Limit max number of items allowed in the "Recent folders & workspaces" list

MR: Pending

Description

In Investigate - Limit `recently.opened` in settings (#389 - closed) we identified an issue that occurs with our end-to-end QA user. These specs use the same user over-and-over again which caused the recently.opened Web IDE settings to grow indefinitely.

Let's add an arbitrary limit to recently.opened so that it's not possible for a user to accidentally blow up their state.

Do we want to add this limit in the Settings Sync API side in the main project, or in the VSCode fork project?

A limit of 100 is added to recently.opened in the settings sync such that this won't grow indefinitely.