fix: Respect show welcome page setting

Description

• This was split from the original MR.
• This MR also prepares the Web IDE to update the base VSCode version to 1.81.X (see this related comment).

Screenshot

Here's a demo showing how we know respect the Show welcome page on startup preference:

Before	After
web_ide_before	web_ide_after

Here's a demo of everything still working with opening the initial file:

• Open Web IDE with "show welcome on startup" unchecked
• Open Web IDE from a specific file that exists
• Open Web IDE from a directory
• Open Web IDE from a file that does not exist

web_ide_file_open_demo