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.toml

@@ -80,6 +80,7 @@ ritz = "0.1.0"
 roblox_install = "1.0.0"
 serde = { version = "1.0.130", features = ["derive", "rc"] }
 serde_json = "1.0.68"
+toml = "0.5.9"
 termcolor = "1.1.2"
 thiserror = "1.0.30"
 tokio = { version = "1.12.0", features = ["rt", "rt-multi-thread"] }