Motivation

• When investigating reports into behaviors of custom expressions like XSS via event filter vega#3984, it would be beneficial to let maintainers rapidly test running vega with the expression interpreter (AST) mode enabled.
• This allows testing extent of the bug without having to set up a local dev environment

Proposal

• Add a checkbox to the settings bar (similar to the tooltip checkbox) which allows toggling this behavior

Out of scope (for now):

• Do not persist this state to URL parameters (as most of the other app states are not URL sync'd either, aside from the VL spec itself). That could be investigated in a follow-up.