QMake: Quote Windows paths containing a plus sign

The cmd.exe copy command treats a plus sign as a concatenation
operator, so "copy /y C:\C++\a.js dest" fails with "The syntax of the
command is incorrect". shell_quote() did not quote arguments that
contain a plus sign, so a project in such a directory could not
be built with the wasm-emscripten spec, which copies qtloader.js
with QMAKE_COPY.

Quote arguments with a plus sign in shellQuoteWin(), and use
shell_quote() for the copy commands in wasm.prf, which passed the
paths unquoted.

Fixes: QTBUG-84023
Change-Id: I6d4bd249d886ffeab5f09b9a2e71e31802f6ec5d
Reviewed-by: Oliver Wolff <oliver.wolff@qt.io>