Add [package.metadata.system-deps] to mlua-sys/Cargo.toml

[slowsage]

Will help in ci environments to know which system dependency to install based on feature.

[alerque]

Distro packager hat on here. How exactly does the system-deps crate go about determining if the appropriate headers and sources are installed on a host system? If this isn't using pkg-config under the hood somewhere (it may be) then this feels like a regression. If it's just an ergonomic wrapper around the same tooling that's probably fine, but it it is some bespoke hand rolled detection system this would be a huge red flag.

[slowsage]

There are two separate changes - adding metadata for parsing by any tool (easier than going through code) and one to use the added metadata for pkg-config's probe() calls:

1. First commit: Specifying dependencies in Cargo.toml in a way that can be parsed. This is for any bespoke tools - no code change, only metadata added to Cargo.toml. Figured the format from system-deps can be reused (as in the second commit, described below). This commit is enough for ci tools to parse and figure out which pkgs to install.
2. Second commit: Once we specify in Cargo.toml, we might as well switch to system-deps in mlua-sys itself. Cargo.toml of system-deps crate uses pkg-config.
   It explicitly uses pkg-config under the hood here.

For my use case, the first commit is enough but system-deps can use it to call pkg-config's probe() and we can avoid repeating the same info in the code, hence the second commit.

Please let me know if I missed something.

[slowsage]

Its ready now. Removed optional = true and changed fallback_names order to match original behavior (fail when no feature is picked and keep old order of fallbacks).

Can make it one commit instead if that is better.
Edit: Not sure about the 1 failure in typos. Did my commits break it?