runtime(m4): update syntax script

runtime(m4): update syntax script

This change does the following to the M4 syntax script:

- In M4 there are no "strings" in the usual sense. Instead, M4 has
  quotes, but the text inside a quoted region is rescanned just like
  outside, and quotes can be nested.
- The old m4String region was misleading and removed. A new m4Quoted
  region reflects proper quoting semantics.
- Removed a duplicate highlight rule.
- Fixed a typo in a highlight group name (m4builtin → m4Builtin).
- Added a reference link to the POSIX M4 specification.
- Removed outdated maintainer URL.

closes: #18192

Signed-off-by: Damien Lejay <damien@lejay.be>
Signed-off-by: Christian Brabandt <cb@256bit.org>