We use calendar versioning which isn't supported by Cargo, so we need
to add an extra leading zero to the month for releases between January
and September to match our usual 'YY.0M' formatting.
Closes#12414
This is a rather large refactor that moves most of the code for
loading, fetching, and building grammars into a new helix-loader
module. This works well with the [[grammars]] syntax for
languages.toml defined earlier: we only have to depend on the types
for GrammarConfiguration in helix-loader and can leave all the
[[language]] entries for helix-core.
