Use `brulee::brulee_activations()` to check for available activation functions in `parsnip::check_args.mlp()`

[christophscheuch]

A recent change in brulee introduced new activation functions and brulee_activations(), which provides available functions (see PR #74). However, parsnip does not support these new activation functions yet, because parsnip::check_args.mlp() has hard coded values (see function definition here).

Would be great to have support for the new activation functions in parsnip.

[hfrick]

Thanks for flagging this @christophscheuch ! We'll get this opened up once the dust of the current work on {brulee} by @topepo has settled a bit.

[topepo]

I am working on this now that I just sent brulee to CRAN (with those features).

[github-actions]

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