gopls: add range over channel postfix completion

This adds a snippet that applies to variables of type chan.

When used, it replaces `channel.range!` with the following snippet:
```
for e := range channel {
   |
}
```
Where `|` indicates the location of the cursor.

Change-Id: I8b2f889b22b9f2c292041e5ca5f63c5d0ca98f11
GitHub-Last-Rev: 9cb894be80d0c5243a5e42779c3e96ba79aa66b5
GitHub-Pull-Request: golang/tools#386
Reviewed-on: https://go-review.googlesource.com/c/tools/+/414194
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>
Reviewed-by: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
gopls-CI: kokoro <noreply+kokoro@google.com>

gopls/internal/regtest/completion/postfix_snippet_test.go

@@ -264,6 +264,27 @@ for k := range foo {
 	keys = append(keys, k)
 }
 
+}
+`,
+		},
+		{
+			name: "channel_range",
+			before: `
+package foo
+
+func _() {
+	foo := make(chan int)
+	foo.range
+}
+`,
+			after: `
+package foo
+
+func _() {
+	foo := make(chan int)
+	for e := range foo {
+	$0
+}
 }
 `,
 		},

internal/lsp/source/completion/postfix_snippets.go

@@ -149,6 +149,14 @@ for {{.VarName .KeyType "k"}}, {{.VarName .ElemType "v"}} := range {{.X}} {
 	{{$keysVar}} = append({{$keysVar}}, {{$k}})
 }
 {{end}}`,
+}, {
+	label:   "range",
+	details: "range over channel",
+	body: `{{if and (eq .Kind "chan") .StmtOK -}}
+for {{.VarName .ElemType "e"}} := range {{.X}} {
+	{{.Cursor}}
+}
+{{- end}}`,
 }, {
 	label:   "var",
 	details: "assign to variables",