Support "gf" in es6 import statements by adding the file extension

[aminroosta]

@johnfn In the es6 import statements, the file extension is omitted, this PR tries to add the file extension automatically.

For example when your cursor is at "../node" the gf command will open ../node.ts

---------current file: src/cmd_line/commands/file.ts---------
import * as node from "../node";

[aminroosta]

@johnfn One test is failing, but it seems irrelevant to changes in this PR.

[johnfn]

Ah, wonderful! Thanks for the PR! :)