Instead of accessing JsonValue struct fields directly, typed
accessors check that the tag matches with the type access.

Drive-by: The factory methods are now static methods on the JsonValue
type itself, making call-sites more readable.

R=tebbi@chromium.org

Bug: v8:8880
Change-Id: I49b37b3ba8eaf1153b8aa93ea08913077c923fdc
Reviewed-on: https://chromium-review.googlesource.com/c/1495559
Commit-Queue: Simon Zünd <szuend@chromium.org>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59968}

Design Doc: https://goo.gl/9G9d9k

The initial prototype consists of a few parts:

The VS Code extension is now built using TypeScript. The build artifact
is checked-in along side the extension. The extension now starts up
the language server when it is activated. The path to the LS
executable is configurable via VS Code settings.

The language server is a separate executable. It adds a light-weight
object model on top of a Json Parser for reading/writing LSP requests
and responses. The current server is very much bare-bones featurewise:
    - Tell the client that the server can handle "goto definition"
    - Recompile when Torque files change
    - Goto definition support for Macros/Builtins, local variables
      and arguments.

R=mathias@chromium.org, mvstanton@chromium.org, tebbi@chromium.org

Bug: v8:8880
Change-Id: Ie9b433e64ee63e9aa757b6bf71e5d52beb15b079
Reviewed-on: https://chromium-review.googlesource.com/c/1494354Reviewed-by: Michael Stanton <mvstanton@chromium.org>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Reviewed-by: Mathias Bynens <mathias@chromium.org>
Commit-Queue: Simon Zünd <szuend@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59960}