snippets: add forrval snippet for single-variable range loops

Add a new snippet, forrval, that generates a for-range loop with a single variable, commonly used with channels and, in Go 1.23+, for value-only iteration over slices and maps.

The snippet expands to:

for v := range v {
}

Updates #4026

Change-Id: Id02283fb9ff6a77b4f76b279ae5196aac0bf0d8b
GitHub-Last-Rev: fa67748
GitHub-Pull-Request: #4027
Reviewed-on: https://go-review.googlesource.com/c/vscode-go/+/770800
Auto-Submit: Madeline Kalil <mkalil@google.com>
Reviewed-by: Madeline Kalil <mkalil@google.com>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>

Author: vinaycharlie01

extension/snippets/go.json

@@ -80,6 +80,11 @@
 			"body": "for ${1:_}, ${2:v} := range ${3:v} {\n\t$0\n}",
 			"description": "Snippet for a for range loop"
 		},
+		"for range single var statement": {
+			"prefix": "forrval",
+			"body": "for ${1:v} := range ${2:v} {\n\t$0\n}",
+			"description": "Snippet for a for range loop with single variable (e.g. channel)"
+		},
 		"channel declaration": {
 			"prefix": "ch",
 			"body": "chan ${1:type}",