Use insertText to add string literal completions

TypeScript Version: 2.8.0-dev.20180125

Code

const x: "a" | "b" = /**/

Expected behavior:

Get "a" and "b" as completions (in addition to global symbols).

Actual behavior:

No useful completions.

Issue from #21372 (comment)

[zardoy]

Ryan Cavanaugh (@RyanCavanaugh) I guess it is not valid anymore?

https://www.typescriptlang.org/play?#code/MYewdgzgLgBAhgLhgcjsmBeFyg

TypeScript/src/services/completions.ts

Lines 2908 to 2910 in 6b43f82

	const literals = mapDefined(
	contextualType && (contextualType.isUnion() ? contextualType.types : [contextualType]),
	t => t.isLiteral() && !(t.flags & TypeFlags.EnumLiteral) ? t.value : undefined);

[jakebailey]

Yeah, this has been supported since at least TS 3.0.