Add CI linting & test matrix across Lua versions

[alerque]

• Lints Lua code
• Lints C code
• Compiles and runs test code across matrix of Lua versions [5.1, 5.2, 5.3, 5.4, luajit, openresty]

[alerque]

Since Actions aren't enabled until they land on master this PR does not reflect what would happen to future PRs that trigger these CI checks, but you can preview what the test outputs look like on my fork here.

[brimworks]

Thank you for the contribution!

[alerque]

It looks like something changed since I submitted this, probably GitHub permissions / CI action versions. I'll look into it and send an update.