Add `startupActions` to Settings UI

[carlos-zamora]

Similar to #8755. These are settings that were added to v1.6 that need to be added to the Settings UI (or include reasoning for why not add them):

Setting	Required?	Notes
startupActions	✔	#8770

This may not be a comprehensive list. @cinnamon-msft is verifying if any are missing and will update this issue accordingly.
EDIT: verified that startupActions is the only one. Updating the issue title appropriately.

Resources:

• Documentation for adding a setting

[DHowett]

This seems a little more advanced, and a late bring-in to get into the settings UI. For it to work properly, we would need to fully parse the commandline string inside the SUI just to make sure it's valid before we let the user commit it. I don't know if we should try to push to get this into 1.6.

[DHowett]

We would probably want the same behavior in the SUI as well, right? That's not 1.6 territory given how close we are to the snap date.

[Don-Vito]

We would probably want the same behavior in the SUI as well, right? That's not 1.6 territory given how close we are to the snap date.

Yes. This is exactly what I was planning to do here 😄

[Don-Vito]

@DHowett - I added the setting itself to UI, however there is an issue adding the parsed command line, as the parsing logic is a part of AppCommandlineArgs, which are is a part of TerminalApp, and thus Settings.Editor is not familiar with it.

Probably we should extract the AppCommandlineArgs into a separate lib and make Editor depend on it? WDYT?