[clang-format] Add MainIncludeChar option. (#78752)

Resolves #27008, #39735, #53013, #63619.

Hello, this PR adds the MainIncludeChar option to clang-format, allowing
to select which include syntax must be considered when searching for the
main header: quotes (`#include "foo.hpp"`, the default), brackets
(`#include <foo.hpp>`), or both.

The lack of support for brackets has been reported many times, see the
linked issues, so I am pretty sure there is a need for it :)

A short note about why I did not implement a regex approach as discussed
in #53013: while a regex would have allowed many extra ways to describe
the main header, the bug descriptions listed above suggest a very simple
need: support brackets for the main header. This PR answers this needs
in a quite simple way, with a very simple style option. IMHO the feature
space covered by the regex (again, for which there is no demand :)) can
be implemented latter, in addition to the proposed option.

The PR also includes tests for the option with and without grouped
includes.