Add Vim syntax highlighting files for Propane

extra/vim/ftdetect/propane.vim

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+au BufNewFile,BufRead *.propane set filetype=propane

extra/vim/syntax/propane.vim

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+" Vim syntax file for Propane
+" Language: propane
+" Maintainer: Josh Holtrop
+" URL: https://github.com/holtrop/propane
+
+if exists("b:current_syntax")
+  finish
+endif
+
+if !exists("b:propane_subtype")
+  let b:propane_subtype = "d"
+endif
+
+exe "syn include @propaneTarget syntax/".b:propane_subtype.".vim"
+
+syn region propaneTarget matchgroup=propaneDelimiter start="<<" end=">>$" contains=@propaneTarget keepend
+
+syn match propaneComment "#.*"
+syn match propaneOperator "->"
+syn keyword propaneKeyword ast ast_prefix ast_suffix drop module prefix ptype start token tokenid
+
+syn region propaneRegex start="/" end="/" skip="\\/"
+
+hi def link propaneComment Comment
+hi def link propaneKeyword Keyword
+hi def link propaneRegex String
+hi def link propaneOperator Operator
+hi def link propaneDelimiter Delimiter