Reorganize definitions.units (#156)

## Changes to `definitions.units`

- Added vim folds repurposing the `!category`/`!endcategory` syntax. There's a modeline at the bottom of the file for this.
- To make the vim folds work better, every single unit now has a category. I also moved them around a little bit so that each category collapses to a single line. This makes the file much easier to navigate. Kind of acts as its own table of contents.
- Rearranged some of the categories, and moved things between categories.
- Adjusted several comments, and added an explanation of the file to the top.
- Deleted some legacy GNU Units directives like !utf8 that Rink will never use. Also several of the function/LUTs that are commented out.
- Uncommented some fractions that apparently GNU Units didn't like. Also uncommented `googol`.
- Moved mercury to the periodic table section.
- Moved the 4 newest elements to be alphabetically sorted.
- Added atomic numbers to every element. (I double checked that the numbers are correct, but I might have still gotten some mixed up...)
- Fixed a few typos and trailing whitespace.

![image](https://github.com/tiffany352/rink-rs/assets/1254344/3d3fbc58-1928-4ded-8612-a84778127c65)

## Changes to rink-core

- Now prints an error on unrecognized directives instead of silently ignoring them.
- Ignores categories for non-units. This may be re-added later but it will require refactoring `Context::categories` to respect namespaces.
- Improved some of the error messages from the units DB loader.