Improve external popup menu appearance

[clason]

• In the TUI, word and menu are printed in separate columns; would be nice to have this for the external menu as well.
• Menu only shows a subset of entries but does not scroll if end is reached.
• Single-character completion items are shown as numbers (their ASCII char codes) instead
  - [ ] Support base64-encoded images in Markdown for the documentation window EDIT(smolck): Not sure how many LSPs (if any?) support this so'll leave it for now.
• Re-enable word-wrapping for multi-line pum (menu) entries, with consistent alignment of all cells (either centered or -- better? -- top-aligned)

[romgrk]

One more thing: in the TUI, completion text is aligned with the input text. It would be nice if the popup could do the same in uivonim.

Actual:

Expected:

[smolck]

@romgrk Should be fixed in 6aee29e, although I'm not sure about how that ends up looking for smaller window sizes.

[romgrk]

One more thing: the popup menu doesn't seem to be respecting Pmenu colors.

[smolck]

One more thing: the popup menu doesn't seem to be respecting Pmenu colors.

This is actually a broader issue due to uivonim kind of doing it's own thing for the colors of external UI elements. I'll see if I can get a PR that fixes this for everything, not just the pmenu.

[smolck]

* [x]  In the TUI, word and menu are printed in separate columns; would be nice to have this for the external menu as well.

* [x]  Menu only shows a subset of entries but does not scroll if end is reached.

* [ ]  Single-character completion items are shown as numbers (their ASCII char codes) instead

* [ ]  Support base64-encoded images in Markdown for the documentation window

* [ ]  Re-enable word-wrapping for multi-line pum (`menu`) entries, with consistent alignment of all cells (either centered or -- better? -- top-aligned)

@clason I think (3) is no longer an issue now that we use node-client, (4) I need a test case for but would theoretically be pretty straightforward to add; also need a test case for that last one if you wouldn't mind, and mind elaborating a bit on what exactly you mean with that one too? Been a while since I worked on the pmenu-related code so don't really remember what that one's about 😅