Enable creating new sketches from a template directory

[gsingh93]

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What kind of change does this PR introduce?

Implements feature request in #2239

What is the current behavior?

Running arduino-cli sketch new MySketch will create a MySketch/MySketch.ino file.

What is the new behavior?

If directories.template is not set, the behavior is the same as before.

If it is set, running arduino-cli sketch new MySketch will copy the directories.template directory to create the MySketch directory.

Does this PR introduce a breaking change, and is titled accordingly?

This isn't a breaking change.

Other Information

Tests/docs haven't been written. Putting this up as a draft to get initial feedback before putting in more effort. One question how to handle setting sketchMainFilePath. Should we rename some file in the template directory to MySketch.ino? Or just pick a .ino from the directory (ideally there's only one) and return that from the RPC call?