Overview

This refactors the syntax and theme assets that are managed and downloaded when building Oatmeal. Configurations now live in a assets.toml file, porting what previously existed in build.rs.

Additionally to the previous variables, each asset now as a nix-hash that's generated by running cargo xtask hash-assets that will later be used to create a Nix package in the official repository.

Lastly, syntaxes and themes can now be configured to be downloaded outside of running cargo build by setting the OATMEAL_BUILD_DOWNLOADED_SYNTAXES_DIR and OATMEAL_BUILD_DOWNLOADED_THEMES_DIR environment variables. When set, build.rs will use files available in those directories rather than downloading the assets defined in assets.toml.