Separate the pkl-core/src/test/files/LanguageSnippetTests directory to another repository

[taichi-ishitani]

I think code snippets in the pkl-core/src/test/files/LanguageSnippetTests are useful assets for other Pkl runtime develpment.
Therefore, I'd like to suggest that this directory is moved to another repository to increase shareability.
By doing so, runtime develpper can import these snippets by adding this separated repository as a submodule and use them for unit testing.

Reference:
https://github.com/json5/json5-tests