runtime(omnimark): update and rewrite syntax script in Vim9 script

Note: this commit rewrites the omnimark syntax script in Vim9 script and
      is therefore probably incompatible with Neovim

closes: #16979

Signed-off-by: Peter Kenny <[email protected]>
Signed-off-by: Christian Brabandt <[email protected]>

Author: kennypete

.github/MAINTAINERS

@@ -562,6 +562,7 @@ runtime/syntax/ninja.vim		@nico
 runtime/syntax/nix.vim			@equill
 runtime/syntax/nroff.vim		@jmarshall
 runtime/syntax/nsis.vim			@k-takata
+runtime/syntax/omnimark.vim @kennypete
 runtime/syntax/ondir.vim		@jparise
 runtime/syntax/opencl.vim		@Freed-Wu
 runtime/syntax/openvpn.vim		@ObserverOfTime