Merge #6488

6488: Textmate grammar: add `+=` to assignment operators r=dustypomerleau a=dustypomerleau

Fixes https://github.com/dustypomerleau/rust-syntax/issues/3.

Co-authored-by: Dusty Pomerleau <dustypomerleau@users.noreply.github.com>

editors/code/rust.tmGrammar.json

@@ -693,7 +693,7 @@
                 {
                     "comment": "assignment operators",
                     "name": "keyword.operator.assignment.rust",
-                    "match": "(-=|\\*=|/=|%=|\\^=|&=|\\|=|<<=|>>=)"
+                    "match": "(\\+=|-=|\\*=|/=|%=|\\^=|&=|\\|=|<<=|>>=)"
                 },
                 {
                     "comment": "single equal",