Adds a VSCode Task (accessible from the "Run Task" menu) for creating
change notes, prompting the user for the language, name, and category of
the change.

The language options presented are based on the existing occurrences of
`change-notes` folders in the repo. There are more such files (in
particular every shared library has a `change-notes` directory), but it
seemed to me that the language change notes are the ones that are most
common, and so in an effort to not clutter the list too much, I only
included the languages.

The selection of categories is based on existing usage -- more
specifically the result of grepping for occurrences of '^category: ' in
the repo. It's possible there are more change categories that could be
added.

Hopefully this should make it more convenient to create change notes
from within VSCode.