FUNC_ATTR_* should only be used in .c files with generated headers.
Defining FUNC_ATTR_* as empty in headers causes misuses of them to be
silently ignored. Instead don't define them by default, and only define
them as empty after a .c file has included its generated header.

Also update the type annotation of TSNode:id(), which returns a string,
not an integer.

Problem:  No test for mode() when executing Ex commands
Solution: Add some test cases and simplify several other test cases.
          Also add a few more test cases for ModeChanged.

closes: vim/vim#13588

https://github.com/vim/vim/commit/fcaeb3d42b228e73c669b2fce78f1d3fe112769f

feat(decoration): allow conceal_char to be a composing char

decor->text.str pointer must go. This removes it for conceal char,
in preparation for a larger PR which will also handle the sign case.

By actually allowing composing chars for a conceal chars, this
becomes a feature and not just a refactor, as a bonus.