support !-functions in plotfunc() and func2type()

[aplavin]

Description

These are internal functions, but still are useful for generic code: stuff like "get the recipe type from a plotting function".
This PR makes them support !-functions as well. Eg, plotfunc(scatter!) === scatter, func2type(scatter!) === Scatter.

Type of change

Delete options that do not apply:

• New feature (non-breaking change which adds functionality)

Checklist

• Added an entry in CHANGELOG.md (for new features and breaking changes)
• Added unit tests for new algorithms, conversion methods, etc.

[aplavin]

bump...
can easily add tests and changelog after confirmation that this change is welcome

[SimonDanisch]

Seems reasonable! Can you add a simple unit test or two? :)

[aplavin]

Added tests and changelog entry, should be ready!

[SimonDanisch]

 Expression: plotfunc(hist!) === hist
  UndefVarError: chopsuffix not defined

Ah yeah, already suspected this isn't 1.6 :(
At some point we may want to drop 1.6, but maybe for now we can just do something that works on 1.6?

[SimonDanisch]

thanks!