prompt user to enable notebook.formatOnCellExecution

[thehappycheese]

This change prompts the user to enable the vscode built in setting notebook.formatOnCellExecution when switching focus to a notebook which is configured to use the 'uiua' language
(see experimental kernel https://github.com/thehappycheese/uiua_kernel)

Given that the kernel is experimental, it may be too early to accept this PR

I am now also wondering:

• The extension should remember if the user refuses, so it can stop prompting
• Maybe there should also be a prompt to enable editor.formatOnSave when editing regular .ua files?
• It would be preferable to only enable this setting for uiua notebooks. Currently changing notebook.formatOnCellExecution or editor.formatOnSave wold apply to all notebooks. I figure this is fine, as long as it is set at the workspace level... but still it is undesirable behavior.
  • I tried to find a way to make this extension listen for notebook cell execution events, but I couldn't find one yet.