Design doc: bit.ly/v8-repl-mode

This CL adds a new REPL mode that can be used via
DebugEvaluate::GlobalREPL. REPL mode only implements re-declaration
of 'let' bindings at the moment. Example:

REPL Input 1: let x = 21;
REPL Input 2: let x = 42;

This would normally throw a SyntaxError, but works in REPL mode.

The implementation is done by:
  - Setting a 'repl mode' bit on {Script}, {ScopeInfo}, {ParseInfo}
    and script {Scope}.
  - Each global let declaration still gets a slot reserved in the
    respective {ScriptContext}.
  - When a new REPL mode {ScriptContext} is created, name clashes
    for let bindings are not reported as errors.
  - Declarations, loads and stores for global let in REPL mode are
    now "load/store global" instead of accessing their respective
    context slot directly. This causes a lookup in the ScriptContextTable
    where the found slot for each name is guaranteed to be the same
    (the first one).

Bug: chromium:1004193, chromium:1018158
Change-Id: Ia6ab526b9f696400dbb8bfb611a4d43606119a47
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1876061
Commit-Queue: Simon Zünd <szuend@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/master@{#64793}