[MusicXML export] Support <part-symbol> export/import for flexible and explicit multi-staff part bracketing

MuseScore should export the part-symbol MusicXML element as a part attribute, in order to (a) capture information about brackets in a multi-staff part, (b) fix situation of pipe organ bracket for manuals that upon re-loading is extended to all three organ staves (see #17661: [MusicXML] Curly bracket covering one, or three staves is missing).

See original forum post for reasoning behind the request.

TODO:
- export
- import
- i/o tests