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: 979739c33c88d1641b63f6f60a17ef57f0ad3f81
GitHub-Pull-Request: golang/vscode-go#3604
Reviewed-on: https://go-review.googlesource.com/c/vscode-go/+/628335
Auto-Submit: Hyang-Ah Hana Kim <hyangah@gmail.com>
Commit-Queue: Hyang-Ah Hana Kim <hyangah@gmail.com>
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>
kokoro-CI: kokoro <noreply+kokoro@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>

extension/syntaxes/go.mod.tmGrammar.json

diff --git a/extension/syntaxes/go.mod.tmGrammar.json b/extension/syntaxes/go.mod.tmGrammar.json
index 37f29a4..e2de6a9 100644
--- a/extension/syntaxes/go.mod.tmGrammar.json
+++ b/extension/syntaxes/go.mod.tmGrammar.json
@@ -65,6 +65,9 @@
 					"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": [
 				{