migrate grammar fetching/building code into helix-loader crate

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.

helix-loader/Cargo.toml

@@ -0,0 +1,23 @@
+[package]
+name = "helix-loader"
+version = "0.6.0"
+description = "A post-modern text editor."
+authors = ["Blaž Hrastnik <blaz@mxxn.io>"]
+edition = "2021"
+license = "MPL-2.0"
+categories = ["editor"]
+repository = "https://github.com/helix-editor/helix"
+homepage = "https://helix-editor.com"
+
+[dependencies]
+anyhow = "1"
+serde = { version = "1.0", features = ["derive"] }
+toml = "0.5"
+etcetera = "0.3"
+tree-sitter = "0.20"
+libloading = "0.7"
+once_cell = "1.9"
+
+# cloning/compiling tree-sitter grammars
+cc = { version = "1" }
+threadpool = { version = "1.0" }