Provide a C library (libpkl) for Pkl

[bioball]

Other language bindings (e.g. Swift and Go) should be able to bind to this library, rather than spawn a child process.

Docs on creating native shared libraries: https://www.graalvm.org/latest/reference-manual/native-image/guides/build-native-shared-library/

[KushalP]

PRs related to this issue:

• C library for Pkl #1026
• Gradle adjustments for libpkl #1081
• Make libpkl tests use AbstractServerTest #1084

As we iterate on this work, changes are being merged into a temporary branch: https://github.com/apple/pkl/tree/c-library