vim-patch:9.0.2098: No filetype support for xcompose files (#25983)

Problem:  No filetype support for xcompose files
Solution: Add filetype detection

closes: vim/vim#13508

4f9074b96c

runtime/syntax/xcompose.vim

@@ -0,0 +1,37 @@
+" Vim syntax file
+" Language:	XCompose
+" Maintainer:	ObserverOfTime <chronobserver@disroot.org>
+" Filenames:	.XCompose, Compose
+" Last Change:	2023 Nov 09
+
+" Comments
+syn keyword xcomposeTodo contained TODO FIXME XXX
+syn match xcomposeComment /#.*/ contains=xcomposeTodo
+
+" Includes
+syn keyword xcomposeInclude include nextgroup=xcomposeFile skipwhite
+syn match xcomposeFile /"\([^"]\|\\"\)\+"/ contained
+syn match xcomposeSubstitution /%[HLS]/ contained containedin=xcomposeFile
+
+" Modifiers
+syn keyword xcomposeModifier Ctrl Lock Caps Shift Alt Meta None
+syn match xcomposeModifierPrefix /\s*\zs[!~]\ze\s*/
+
+" Keysyms
+syn match xcomposeKeysym /<[A-Za-z0-9_]\+>/
+syn match xcomposeKeysym /[A-Za-z0-9_]\+/ contained
+syn match xcomposeString /"\([^"]\|\\"\)\+"/ contained nextgroup=xcomposeKeysym skipwhite
+syn match xcomposeColon /:/ nextgroup=xcomposeKeysym,xcomposeString skipwhite
+
+hi def link xcomposeColon Delimiter
+hi def link xcomposeComment Comment
+hi def link xcomposeFile String
+hi def link xcomposeInclude Include
+hi def link xcomposeKeysym Constant
+hi def link xcomposeModifier Function
+hi def link xcomposeModifierPrefix Operator
+hi def link xcomposeString String
+hi def link xcomposeSubstitution Special
+hi def link xcomposeTodo Todo
+
+let b:current_syntax = 'xcompose'