check-style: Add check for ns-3 #include headers with angle brackets

Add a check to check-style-clang-format.py to detect ns-3 #include headers that use angle brackets and convert them to quotes.

The issue of using angle brackets (<>) vs. quotes ("") is being discussed in #795 and !886. While no consensus has been found yet, I believe we should, at least, harmonize the codebase on a single style. This allows a better readability of the code, especially when used together with the other checks by clang-format and the include prefixes in !1640 (merged).

I have also updated the coding style manual to reflect this change.

If the decision is to standardize on angle brackets, then this MR can be easily refactored to do the opposite.

For reference, a more detailed discussion about this topic was carried out in !886.

Close !886.