Add a command for quickly creating a new extension

[zadjii-msft]

This adds a new form page for quickly creating a new extension.

• ExtensionTemplate/ should be zipped up into template.zip.
• fill in that form, then when you submit the form, we'll unzip the template.
• You should be able to just open that solution up and just go
• I moved the built-in commands lower in the list. The only visible commands it exposes are "Open CmdPal settings" and "Create new extension" (the others are hidden fallbacks, so it doesn't really matter)
• To mitigate that the settings command is lower in the list, I added it to the "page title" spot in the command bar (only on the root view).

Closes #311