Add variants of DString.insert_at to match .append

[chri-k]

Adds more methods to insert things in the middle of a DString:

insert_string_at(usz, DString)
insert_char_at(usz, char)
insert_char32_at(usz, Char32)
insert_utf32_at(usz, Char32[])

And to match .append, i renamed .insert_at(usz, String) to insert_chars_at, and instead made .insert_at a macro that picks a function based on type, like .append (it will pick the renamed one for String so nothing should break).

I also updated append_utf32 and append_char32 to now return the size of the utf8 they appended for convenience (and of course the insert versions do the same). That shouldn't affect anything either

[chri-k]

And of course i somehow managed to include six lines of space indentation in the PR, despite already having undone the commit once to fix those exact same six lines of space indentation.

[lerno]

Can you add something in releasenotes about this as well please?

[chri-k]

My bad. Added line to release notes (and fixed the spaces this is embarrassing)

[lerno]

Thank you!