Save theme edits from the site editor to the theme files #39194
Description
Since the site editor can export a theme's modifications, it makes sens to have the ability to save (and hence alter) the current theme's files when edits have been made in the site editor.
However the goal is not to create the whole "file editor" thing again :) but to enable designers to visually update the theme. Therefore I suggest that this functionality be available only while theme.json has a "development_mode:true" flag. When that flag is unset the modifications can only reside in the database.
When editing a theme if said flag is true, we should have a new button next to "Save", or maybe a split button, with an optional "Update theme files". Clicking this button would enter a flow similar to the entity saving flow where all the files that will be updated are listed, and the user can choose which to not update.