This is a bit of a micro-optimization: in the current setup we waste
a thread in the pool for a local grammar only to println! a message
saying we're skipping fetching because it's a local grammar.
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.
