modem: chat: Add synchronous functions for script exec

This commit adds one new feature, modem_chat_run_script(),
which synchronously runs a script.

The existing function modem_chat_script_run() is async,
and doesn't follow the naming convention created for the
other modem modules, specifically, all functions are sync,
unless appended with _async.

To preserve backwards compatibility, the existing function
modem_chat_script_run() will remain until deprecated. It
simply calls modem_chat_run_script_async().

Signed-off-by: Bjarki Arge Andreasen <bjarkix123@gmail.com>