Add HTTP headers parsing

[alexkunin]

This is far from the final implementation. More of a conversation starter.

Few notes:

1. It uses http.ReadResponse to properly parse headers.
2. Due to p.1, headers are re-ordered, their names are canonicalized.
3. Body is also parsed by http.ReadResponse because that's how it works.
4. Special handling of curl's status line HTTP/2 (no minor version) is included.
5. User will see an array with 3 values: status line, headers object, parsed body. I mean, fx is JSON viewer, so, we convert things to JSON and view them.
6. Headers with multiple values are presented as arrays.
7. No JavaScript part whatsoever for now, Go part requires lots of attention.

Things to consider:

1. What if body is not a valid JSON? Currently it will fail. But we could show it as a string instead.
2. Body is just brute-force parsed as JSON, Content-Type and other related headers are ignored. What if it's not application/json, or not UTF-8? Or if it's YAML?
3. http.ReadResponse might be picky about Content-Length header (especially wrong values). Just believe it will work fine? Make it ignore content length? Show some warning?
4. Apparently, if we want to see original header names (non-canonicalized), in the original order, we should find an alternative to http.ReadResponse. Is this really important? Maybe users will want a CLI option (or hotkey) to switch modes?
5. Often field values are valid JSON, should we do something about it? E.g.:

report-to: {"group":"heroku-nel","max_age":3600,"endpoints":[{"url":"https://nel.heroku.com/reports?ts=1710712651&sid=c4c9725f-1ab0-44d8-820f-430df2718e11&s=jGCtfLAslGLM1QUWfRhiMIJr5mcM29gDwJ5JsYNYRuA%3D"}]}
reporting-endpoints: heroku-nel=https://nel.heroku.com/reports?ts=1710712651&sid=c4c9725f-1ab0-44d8-820f-430df2718e11&s=jGCtfLAslGLM1QUWfRhiMIJr5mcM29gDwJ5JsYNYRuA%3D
nel: {"report_to":"heroku-nel","max_age":3600,"success_fraction":0.005,"failure_fraction":0.05,"response_headers":["Via"]}

closes #290

[antonmedv]

I think displaying in this format will be much cooler:

[antonmedv]

And let's use our own parser jsonx

[alexkunin]

I was using encoding/json because I needed to manipulate JSON (mostly combine), not only parse. But I agree, having two separate parsers is a bit excessive.

[alexkunin]

I think displaying in this format will be much cooler:

I don't disagree, but this format rises questions:

1. Is it interactable?
2. If yes, how yanking should work?
3. If no, what if terminal size is too small to show all headers?

I'd say the following will pose least problems:

1. Make it interactable: let it render as current JSON structure (object + strings or arrays of strings as values), but don't display any brackets or indent. This should be rather easy.
2. On yank:

• if cursor is inside headers, work with headers JSON only
• if cursor inside body, work with body JSON only

So, effectively:

1. Need to implement ability to have two consecutive JSON documents.
2. Add special document renderer without brackets.

[antonmedv]

Is it interactable?

Yes.

If yes, how yanking should work?

I guess this will be just a key/value pair.

If no, what if terminal size is too small to show all headers?

Wrapping and scrolling.

Need to implement ability to have two consecutive JSON documents.

Yes. This is actually number one thing what is needed.

Add special document renderer without brackets.

This is easy.

[alexkunin]

This update is much more finished:

1. Simple regexp-based HTTP response parser (basically, everything is retained as close to original as possible):
   • original order of headers retained
   • header names are displayed as-is (no canonicalization)
   • multiline header values are preserved (and later displayed) as separate lines
   • multiple instances of same header are not combined
   • all of this as a neat small package with own tests
2. New keys in theme (currently header name has same color as object key, and header name + status have same color as string). Note: that differs from your example screenshot because it's somewhat hard for me to come up with colors for every theme available. Anyhow, that's the smallest of problems, considering how large this PR is now.
3. Yanking/searching should work fine.
4. Got rid of encoding/json.

Re multiple documents support, I've tried that approach at first, but it requires too many changes in lots of places. So, instead I'm just adding some new nodes at the start to display headers (along with new Node.Kind field which simplifies things alot).

[antonmedv]

Awesome! Let me review and test this PR before merging.

[alexkunin]

Yay! But FYI, currently I have no idea what JavaScript part of the project does, so, this PR so far was about Go code only.