make SCI registers readable and make plugins installable

[CelliesProjects]

I opened this PR to be able to read the SCI status registers.

[CelliesProjects]

I expanded the scope of this PR to make all *.plg files installable.

[CelliesProjects]

@baldram
I started this as a poc/test to let the decoder decide what codec is running. That works fine now.
That part needs readRegister() public.

The later changes are required so that 16 bit flac files can be decoded and recognised by my stream library.

[CelliesProjects]

@baldram Could you please review these changes?

[baldram]

@CelliesProjects Thank you for letting me know that this change is complete and the POC/test phase has been wrapped up. Will 👀

[baldram]

Thank you @CelliesProjects! Looks good. We will need to add a proper description to the commit.

Theoretically there is a low-impact breaking change: PLUGIN_SIZE macro renamed to
per-patch constants. Users should replace with PATCHES_FLAC_SIZE, PATCHES_FLAC_LATM_SIZE,
PATCHES_LATM_SIZE, or PATCHES_PITCH_SIZE as appropriate.

[baldram]

@CelliesProjects Are you able to use it directly from master without me releasing a new version?

[CelliesProjects]

@baldram Thanks for the quick merge!

I think a new release might be in place. Very convenient for novice users.