Downgraded Ubuntu VS version for Windows the build

For whatever reason compiling Windows native binaries on Ubuntu 22.04 produces a 32-bit binary that doesn't work (the 64-bit one does work).
Compiling them on Ubuntu 20.04 produces working binaries, which are also both much smaller than the ones from Ubuntu 22.04.

Added workflow run for PRs as a CI check

Merge pull request #2 from RoosterDragon/runtime-metamethod-cache

Cache metamethods in each LuaRuntime.

Merge pull request #1 from RoosterDragon/cache-metamethods

Cache metamethods, finalize references

Add ILuaToStringBinding so that CustomClrObjects can implement __tost…

…ring.