Create highlight group for string delimiters

Some users wish to highlight string delimiters (e.g., quotation marks)
in a different color from the body of the string itself. This feature is
provided by several other languages' syntax highlighting rules, and is
easy to achieve for Rust as well.

Add a match group for the string delimiters and by default, assign it to
the same highlight as the rest of the string, to preserve existing
behavior. Users who wish to make use of this feature need only adjust
their color scheme files to link the new rustStringDelimiter group to their
preferred syntax highlighting group.

Author: bk2204

syntax/rust.vim

@@ -144,9 +144,9 @@ syn match     rustEscapeError   display contained /\\./
 syn match     rustEscape        display contained /\\\([nrt0\\'"]\|x\x\{2}\)/
 syn match     rustEscapeUnicode display contained /\\u{\%(\x_*\)\{1,6}}/
 syn match     rustStringContinuation display contained /\\\n\s*/
-syn region    rustString      start=+b"+ skip=+\\\\\|\\"+ end=+"+ contains=rustEscape,rustEscapeError,rustStringContinuation
-syn region    rustString      start=+"+ skip=+\\\\\|\\"+ end=+"+ contains=rustEscape,rustEscapeUnicode,rustEscapeError,rustStringContinuation,@Spell
-syn region    rustString      start='b\?r\z(#*\)"' end='"\z1' contains=@Spell
+syn region    rustString      matchgroup=rustStringDelimiter start=+b"+ skip=+\\\\\|\\"+ end=+"+ contains=rustEscape,rustEscapeError,rustStringContinuation
+syn region    rustString      matchgroup=rustStringDelimiter start=+"+ skip=+\\\\\|\\"+ end=+"+ contains=rustEscape,rustEscapeUnicode,rustEscapeError,rustStringContinuation,@Spell
+syn region    rustString      matchgroup=rustStringDelimiter start='b\?r\z(#*\)"' end='"\z1' contains=@Spell
 
 " Match attributes with either arbitrary syntax or special highlighting for
 " derives. We still highlight strings and comments inside of the attribute.
@@ -286,6 +286,7 @@ hi def link rustEscapeUnicode rustEscape
 hi def link rustEscapeError   Error
 hi def link rustStringContinuation Special
 hi def link rustString        String
+hi def link rustStringDelimiter String
 hi def link rustCharacterInvalid Error
 hi def link rustCharacterInvalidUnicode rustCharacterInvalid
 hi def link rustCharacter     Character