* Fix the type declaration of `!!raw-loader!` modules.
Instead of declaring them to export an object with a single key
`default` which is a string, the modules have a default export, which
is a string.
Note, that this doesn't actually change the generated code, just the
types that typescript sees. The code worked before because the only
thing done to the values was to coerce the values to a string, which
turned into a no-op.
* Switch from using `raw-loader` to using a source asset module.
`raw-loader` was deprecated in webpack v5.
This ports the PDF manual by @catloversg into .md form, so it can be used in-game and via browsing on GitHub.
It also adds MathJax rendering to in-game docs, so that the math formulas can be rendered. (GitHub already supports this natively.)
