Add PklProject filename to Pkl

[nihaals]

Add PklProject to Pkl's filenames.

Checklist:

• I am adding a new extension to a language.
  • The new extension is used in hundreds of repositories on GitHub.com
    • Search results for each extension:
      • https://github.com/search?type=code&q=NOT%20is%3Afork%20path%3A*PklProject%20amends
  • I have included a real-world usage sample for all extensions added in this PR:
    • Sample source(s):
      • https://github.com/apple/pkl-swift/blob/4ef24e1ffac624de9bd6909a5a830f08f48fe38a/codegen/src/PklProject
    • Sample license(s):
      • Apache-2.0
  • I have included a change to the heuristics to distinguish my language from others using the same extension.

[lildude]

• I have included a real-world usage sample for all extensions added in this PR:

You haven't. You need to actually add the files to the PR and also ensure the tests pass with the new filenames.

[nihaals]

Oh it's referring to samples/ not just example links, will add them now.