Option to bundle essential externals.

[alisomay]

Make a list of externals which modern pd users use commonly and bundle them with the library.

Also we can make this a cargo feature.

This is a complicated design choice since externals need to be statically linked with libpd.

• Would it be better to include them as submodules?
• Would it better to vendor pre-built ones and update them time to time.
• Should we give examples and advise people to fork libpd-sys to include externals?

Maybe there can be more ideas..