Fix extension activation failing if no folder is opened

[Christellah]

Description:

We now only allows the user to use the serial monitor if they have a folder open, because the file cpx.json can only be created in a workspace. This way we let the user use the extension without a folder but they'll receive an error if they try to use a serial monitor related command.
(Fixing BUG:32887)

Type of change

• Bug fix (non-breaking change which fixes an issue)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Limitations:

In the future, it would be interesting to let the user use the serial monitor even if they don't have a folder opened or just a file opened.

Testing:

• Try running serial monitor commands with no folder opened (with a file and without a file opened) : it should prompt you a popup
• Try running serial monitor command with a folder open and make sure it's still working properly

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• Any dependent changes have been merged and published in downstream modules

[jonathanwangg]

Tested it out and works as described. Code looks great as well!