[lex.pptoken] "Whitespace" and "whitespace characters" should be definitions

[Eisenwave]

The index points to https://eel.is/c++draft/lex.pptoken#1 as the place where "whitespace" is introduced.

The terms "whitespace" and "whitespace character" are quasi-definitions within that paragraph, and they should be formatted as such (i.e. in italics).

There are many places where [lex] refers to whitespace, so by all means, it should be a defined term.

[Eisenwave]

This issue is essentially obsoleted if cplusplus/papers#2287 is accepted. Even if not, it's worth keeping in mind that we could do something editorially here.

Also worth keeping in mind that we may want to remove the index entry for "whitespace" once whitespace-character is a formal term.

[AlisdairM]

See also #7359, https://wg21.link/P2348, and cplusplus/papers#1027, although naturally I prefer my own paper :)