feat: improve css handling

[shadows-withal]

Ref: #610.

Changes the logic in the build.rs to allow for a more flexible way of controlling how CSS should be built. Most importantly, this reverts the behaviour where oranda would download a Tailwind binary on build - this functionality is now hidden behind an environment variable. You can now control how CSS is built via the following ways:

• Set ORANDA_USE_TAILWIND_BINARY to build CSS at runtime (in development), or at build time (in release mode)
• Prebuild the CSS using npm run build in oranda-css/, and on the next build, oranda will embed the built CSS file into the binary.
• If you do neither of these, oranda will now fall back to fetching from GitHub releases.

Breaking changes

Distributors that would previously rely on Tailwind to build CSS for embedding into the binary now need to prebuild CSS using npm run build, or they need to use their own Tailwind binary to produce a oranda-css/dist/oranda.css file.

TODO

• Update comments in css.rs
• Create new docs page for building oranda, explain in detail the different build cases
• Update contributing docs with new instructions
• Re-add package.json