Report improvements

[mds1]

Closes #33, replaces #67

Why we are closing #67 in favor of this approach:

• The original plan was to update proposal checks to output generic results, independent of formatting, then have postprocessors to convert that to markdown, PDF, HTML, or whatever other format.
• There's no standardized format I came across for doing this, so we were going to use a version of pdfmake's type definition as the format since it was high-level and seemed like a good option.
• As we implemented the postprocessors, it became a decent amount of work with various edge cases, etc. In general it felt like we were re-inventing the wheel.
• @wildmolasses pointed out that if we just output markdown, we can use tools like remark and md-to-pdf which let you convert markdown to an AST and convert to HTML/other formats from there, as well as edit the AST to modify content, etc
• This means proposal checks are still expected to output their results as markdown, so helper methods will be added to simplify this.

Actions now result in 3 artifacts per report: A markdown file, a PDF, and an HTML file. The collapsible details blocks supported by markdown don't work well with the other output formats, so they have been removed and replaced with a table of contents for easier navigation.

Sample proposal reports can be found in the artifacts of this workflow run: https://github.com/Uniswap/governance-seatbelt/actions/runs/2515723372

The HTML reports have two issues:

• Table of contents links do not work. It seems the emojis cause issues here, tracking this in reports: fix links in HTML reports #70 Fixed by Get toc links working in all 3 report formats #72
• The HTML reports just use the basic, default styling, and could use some styling to look better. Tracking in reports: Improve HTML report styling #71

[mds1]

This file was renamed from markdown.ts

[mds1]

Successful test CI run in https://github.com/Uniswap/governance-seatbelt/actions/runs/2569653666, so merging this