gopls: add string join/split postfix completions

This adds two new snippets, centred around string manipulation.

The first snippet applies to variables of type string, and is called
split. When used, it replaces `someStringVar.split!` with
`strings.Split(someStringVar, "|")`, where `|` indicates the location of
the cursor.

The second snippet is essentially the same as the first, but in
"reverse". Meaning that rather than going from a string to a slice of
strings, it goes from a slice of strings to a single string:
`someStringSlice.join!` -> `strings.Join(someStringSlice, "|")`.

Change-Id: I0e303a39766463034687f76a5d9dbab419e2021b
GitHub-Last-Rev: 935cc418e75d54f0e5b1194cdb576c8829252a26
GitHub-Pull-Request: golang/tools#347
Reviewed-on: https://go-review.googlesource.com/c/tools/+/362474
Reviewed-by: Robert Findley <rfindley@google.com>
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
gopls-CI: kokoro <noreply+kokoro@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Trust: Hyang-Ah Hana Kim <hyangah@gmail.com>

gopls/internal/regtest/completion/postfix_snippet_test.go

@@ -372,6 +372,44 @@ func _() {
 }
 `,
 		},
+		{
+			name: "string split",
+			before: `
+package foo
+
+func foo() []string { 
+	x := "test"
+	return x.split
+}`,
+			after: `
+package foo
+
+import "strings"
+
+func foo() []string { 
+	x := "test"
+	return strings.Split(x, "$0")
+}`,
+		},
+		{
+			name: "string slice join",
+			before: `
+package foo
+
+func foo() string {
+	x := []string{"a", "test"}
+	return x.join
+}`,
+			after: `
+package foo
+
+import "strings"
+
+func foo() string {
+	x := []string{"a", "test"}
+	return strings.Join(x, "$0")
+}`,
+		},
 	}
 
 	r := WithOptions(Options(func(o *source.Options) {

internal/lsp/source/completion/postfix_snippets.go

@@ -174,6 +174,18 @@ for {{.VarName .KeyType "k"}}, {{.VarName .ElemType "v"}} := range {{.X}} {
 	body: `{{if and (eq .Kind "tuple") .StmtOK -}}
 {{.Import "fmt"}}.Println({{.X}})
 {{- end}}`,
+}, {
+	label:   "split",
+	details: "split string",
+	body: `{{if (eq (.TypeName .Type) "string") -}}
+{{.Import "strings"}}.Split({{.X}}, "{{.Cursor}}")
+{{- end}}`,
+}, {
+	label:   "join",
+	details: "join string slice",
+	body: `{{if and (eq .Kind "slice") (eq (.TypeName .ElemType) "string") -}}
+{{.Import "strings"}}.Join({{.X}}, "{{.Cursor}}")
+{{- end}}`,
 }}
 
 // Cursor indicates where the client's cursor should end up after the