Rename no_tabs_linter() to whitespace_linter()

[MichaelChirico]

Inspired by r-devel/rdevguide#133 -- I know base R has a lot of places where \t is used, so adding flexbility to the linter will help.

The new suggested name is also more stylistically similar to other linters in the suite.

This issue is just about renaming the linter, filing now to get in before the 3.1.0 "deadline" alongside the other renamings. Future issues will address extending flexibility. Not doing so here because it's not trivial -- setting \t as the desired whitespace probably just refers to initial indentation, with other spaces in the file still just . The current linter doesn't support this flexibility (it just looks for \t anywhere), so it's more of an investment to get there.