vim-patch:9.1.1821: filetype: Not all PKL files are recognized (#36008)

Problem:  filetype: Not all PKL files are recognized
Solution: Detect *.pcf as pkl filetype, detect using the pkl-lsp://
          protocol as pkl filetype, include PKL syntax script
          (Jan Claußen)

This adds basic syntax support for the new PKL language by Apple.

What works:
- Shebang support
- Comment support
- Integers (decimal, hex, octal and binary) support
- Floating point support including exponentials
- Basic datatype support
- Unicode escape delimiters
- Escape code support
- String interpolation
- Support up to five pounds for custom delimiters
- Folding of multi-line comments and blocks

What doesn't work:

  The language heavily uses parameterized type declarations, which can get
  very complex. It is very hard to highlight this properly. There is
  official Tree-sitter support for this. Since it is hard to pull this off
  in a vim syntax file, I opted for basic support of the data types.

References:
  https://github.com/apple/pkl-pantry

fixes: vim/vim#18271
closes: vim/vim#18274

67a8f2945e

Co-authored-by: Jan Claußen <jan.claussen10@web.de>

test/old/testdir/test_filetype.vim

@@ -612,7 +612,7 @@ func s:GetFilenameChecks() abort
     \ 'pilrc': ['file.rcp'],
     \ 'pine': ['.pinerc', 'pinerc', '.pinercex', 'pinercex'],
     \ 'pinfo': ['/etc/pinforc', '/.pinforc', 'any/.pinforc', 'any/etc/pinforc'],
-    \ 'pkl': ['file.pkl'],
+    \ 'pkl': ['file.pkl', 'file.pcf'],
     \ 'pli': ['file.pli', 'file.pl1'],
     \ 'plm': ['file.plm', 'file.p36', 'file.pac'],
     \ 'plp': ['file.plp'],