extension: fix go.mod retract block syntax highlight

retract block in go.mod can contain ranges in the form of "[vStart, vEnd]", which is not currently well-highlighted

Change-Id: I8f2a41b6fc9a268c939e0e65fa82b9d2146ea360
GitHub-Last-Rev: 979739c
GitHub-Pull-Request: #3604
Reviewed-on: https://go-review.googlesource.com/c/vscode-go/+/628335
Auto-Submit: Hyang-Ah Hana Kim <[email protected]>
Commit-Queue: Hyang-Ah Hana Kim <[email protected]>
Reviewed-by: Hyang-Ah Hana Kim <[email protected]>
kokoro-CI: kokoro <[email protected]>
Reviewed-by: Robert Findley <[email protected]>

Author: frankli0324

extension/syntaxes/go.mod.tmGrammar.json

@@ -64,6 +64,9 @@
 				{
 					"include": "#semver"
 				},
+				{
+					"include": "#semver_range"
+				},
 				{
 					"include": "#unquoted_string"
 				}
@@ -142,6 +145,15 @@
 			"match": "v(?:0|[1-9]\\d*)\\.(?:0|[1-9]\\d*)\\.(?:0|[1-9]\\d*)(?:-[\\da-z-]+(?:\\.[\\da-z-]+)*)?(?:\\+[\\da-z-]+(?:\\.[\\da-z-]+)*)?",
 			"name": "constant.language.go.mod"
 		},
+		"semver_range": {
+			"begin": "\\[",
+			"patterns": [
+				{
+					"include": "#semver"
+				}
+			],
+			"end": "\\]"
+		},
 		"string_escaped_char": {
 			"patterns": [
 				{