[vscode] Pass experiments to fidlc

Update the LSP plugin to pass a configurable set of experiments to
fidlc. The plugin was previously hardcoded to set allow_new_syntax. Now
allow_new_syntax has been moved to a vscode configuration option. This
grants increased flexibility as changing the expeiments allowed in fidlc
will not automatically break the FIDL plugin (users can now manually
disable old experiments if they get removed before we take them out of
the plugin), and it allows devs using FIDL to try out newer experiments
before they are enabled by default in fidlc.

Bug: 88366
Change-Id: If47d211adac483f3c59f95efd2f5f47570c8350f
Reviewed-on: https://fuchsia-review.googlesource.com/c/fidl-misc/+/753242
Reviewed-by: Alex Zaslavsky <azaslavsky@google.com>