Generate a default mbed_app.json with "mbed new" to increase config system awareness

[0Grit]

See handbook-#462

It's also one less step the user needs to complete themselves. Step being, lookup name of file to create, mbed_app.json, and the basic syntax to follow.

[0Grit]

@theotherjimmy does this belong in mbedOS?

Also, I'm finally starting to lock into my brain the names of mbed_app.json, mbed_lib.json, and the json object sections that go within them. All the previous syntaxes for yotta and whatever else was used since then weren't helping.

However I still consistently need to look up the names of sections.
This enhancement would save me a lot of time, confusion and allow me to onboard new developers more quickly.

[theotherjimmy]

@loverdeg-ep This belongs in Mbed CLI.

This enhancement would save me a lot of time, confusion and allow me to onboard new developers more quickly.

Agreed, this is probably a good idea.

[ciarmcom]

Internal Jira reference: https://jira.arm.com/browse/MBOCUSTRIA-58