DocumentSymbol responses provide all symbols (macros, classes, etc.)
in a given document. The LSP standard evolved over time and supports
two different kind of responses here:
  - A simpler one, that is a plain list of all the symbols
  - A more detailed one, allowing a hierarchy of symbols. For example
    a class symbol has a list of field/method symbols attached. This
    is used by editors to build hierarchical outline views.

For now, the simpler response is chosen as its also used for
workspace wide symbol searches.

This CL adds the necessary boilerplate so the actual implementation CL
is easier to review.

R=tebbi@chromium.org

Bug: v8:8880
Change-Id: I6c86fc839b1f4e0309f6403a5f9afd5c162c0e89
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1598757
Auto-Submit: Simon Zünd <szuend@chromium.org>
Commit-Queue: Tobias Tebbi <tebbi@chromium.org>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#61294}