patch 9.1.1821: filetype: Not all PKL files are recognized

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: #18271
closes: #18274

Signed-off-by: Jan Claußen <jan.claussen10@web.de>
Signed-off-by: Christian Brabandt <cb@256bit.org>

.github/MAINTAINERS

@@ -585,6 +585,7 @@ runtime/syntax/pbtxt.vim				@lakshayg
 runtime/syntax/pdf.vim					@tpope
 runtime/syntax/perl.vim					@petdance
 runtime/syntax/php.vim					@TysonAndre
+runtime/syntax/pkl.vim					@jclsn
 runtime/syntax/plsql.vim				@lee-lindley
 runtime/syntax/pod.vim					@petdance
 runtime/syntax/poefilter.vim				@ObserverOfTime