ENH Add missing base.is_clusterer() function

[ChVeen]

Reference Issues/PRs

Fixes #28960

What does this implement/fix? Explain your changes.

This PR proposes to add the missing base.is_clusterer() function analogously to base.is_classifier().
There is a user demand for this as can be seen in discussion #28904.
The missing unit test for base.is_regressor() is added as well.

Any other comments?

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[adrinjalali]

Since this is not introducing clusterer as a new thing, and it already exists, I'm okay with it.

[adrinjalali]

Needs a changelog introducing the new function.

[ChVeen]

Needs a changelog introducing the new function.

Thanks for the hint. Added to changelog...

[ChVeen]

BTW: I added this new function to the changelog of version 1.5.
If this PR is about to be merged after releasing 1.5, I've to shift the changelog entry to the new version 1.6.

[adrinjalali]

Yes, the changelog would need to be moved.

[ChVeen]

Moved the changelog entry from version 1.5 to 1.6.

[glemaitre]

We should also add the function into the API reference in the file doc/modules/classes.rst under the function of the module sklearn.base.

[glemaitre]

LGTM. Thanks @ChVeen

[Charlie-XIAO]

Do we need the versionadded thing when adding new API?

[Charlie-XIAO]

LGTM, I'm enabling auto-merge. Thanks @ChVeen!