Add PReLU Activation

[isaaccorley]

What does this PR do?

• adds flax.linen.activation.PReLU discussed in PReLU activation implementation #1419.
• adds tests/linen/linen_activation_test.py
• adds PReLU to docs

Fixes #1419

Checklist

• This PR fixes a minor issue (e.g.: typo or small bug) or improves the docs (you can dismiss the other
  checks if that's the case).
• This change is discussed in a Github issue/
  discussion (please add a
  link).
• The documentation and docstrings adhere to the
  documentation guidelines.
• This change includes necessary high-coverage tests.
  (No quality testing = no merge!)

[codecov-commenter]

Codecov Report

Merging #1570 (73b9e0c) into main (136f41a) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1570      +/-   ##
==========================================
+ Coverage   83.07%   83.10%   +0.02%     
==========================================
  Files          69       69              
  Lines        5826     5836      +10     
==========================================
+ Hits         4840     4850      +10     
  Misses        986      986              

Impacted Files	Coverage Δ
flax/linen/__init__.py	100.00% <ø> (ø)
flax/linen/activation.py	100.00% <100.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 136f41a...73b9e0c. Read the comment docs.

[jheek]

Looks good overall, I just have 2 small comments

[jheek]

Thanks, Looks good to me!