Change naming to alt.param and alt.add_params

[ChristopherDavisUCI]

I believe this pull request incorporates the naming changes suggested by @mattijn in #2598. I didn't come across anything subtle in making these changes; I mostly just used Find and Replace. Please let me know if there are aspects that need to be thought through more carefully!

Needless to say, this will break the code of anyone currently using alt.parameter or alt.add_parameter.

[mattijn]

Nice! The linting check presents one issue to be resolved: https://github.com/altair-viz/altair/runs/7285808845?check_suite_focus=true

The conflict is around here: https://github.com/altair-viz/altair/pull/2656/files#diff-bbe4e187b18d242a366c820d023afd89041759cb96e4ec66c3f34559a72c2f9dR2229.
Can you solve this issue?

It's understood that this will affect code in the wild of people already trying out the main branch. Therefore thanks again for working on this!

[ChristopherDavisUCI]

Thanks @mattijn. Weird that the search-and-replace led to wrong formatting. (But then there was also a mistaken parameter still around that the flake8 check found.) I think those issues are corrected now.

[mattijn]

Nice!