Cleanup codebase, docs and examples with American spelling

[damusss]

I used Visual Studio Code search tool to find all the occasions for non American spelling, looked quickly at each one and chose to replace them or not.
I ignored file names, files, links and the C/PY color dict files (where both spellings are intended)
The reason is:

• the codebase should use one convention and stick to it
• the American spelling is preferred to match the actual pygame classes and methods such as Color and grayscale()

For now I considered the following non American spelling (you can request more):

• grey -> gray
• colour -> color

If all checks pass it means I didn't break any variable name.

[ankith26]

LGTM, thanks for the PR 👍

[oddbookworm]

LGTM!