Consistently use `code point` instead of sometimes saying `codepoint`

[svgeesus]

One minor point I would consult with i18n is mixing use of code point(s) and codepoint(s). Former should be our desire per glossary, but both are used in the document, such as codepoints in section 3.1 or code points in section 5.2.1.1 (or detailed per step processing definitions). Should we point this? (note, there is one parameter in structure named as codepoints, so some of these are correct as referencing that)

Originally posted by @himorin in https://github.com/w3c/i18n-request/issues/238#issuecomment-2273974295

[svgeesus]

There are 6 uses of code point and 42 uses of codepoint.

(parameters in structures should not be renamed, of course; this is about the prose)

[garretrieger]

I'll go ahead and update all uses to "code point(s)"