Can the inputbox show a red outline for invalid filenames?

[rzhao271]

Testing #155731

Ref #155731 (comment)

When the user types in an invalid file name,

• The inputbox can show a red outline with an error message
• The save file dialog shouldn't open when the user presses enter while they're still focused on the inputbox and what they typed is invalid

I'm not sure what to do about the "New file (filename)" quickpick option when the file name is invalid, though.

CC @Yoyokrazy