Add support for TOML files (#633)

TOML maps well to Lua, is easier to read and write than JSON, and is
commonly used by Roblox tools.

Use cases:
* Put game, plugin, or library config in a toml file
* Sync in toml files generated by tools
* Sync in config files for tools so that the game can double-check that
the config file has been followed. (e.g. check that packages match
versions specified in wally.toml)

Cargo.lock

@@ -1922,6 +1922,7 @@ dependencies = [
  "termcolor",
  "thiserror",
  "tokio",
+ "toml",
  "tracy-client 0.13.2",
  "uuid",
  "walkdir",