Auto-completion of macros in configuration forms

Somewhat related to ZBXNEXT-1517, but actually inspired by auto-completion of LLD macros being discussed in the context of ZBXNEXT-4087.

Basically, whenever user types in ‍{ in any field of any configuration form that supports any kind of macros (built-in macros, user macros, LLD macros, {host:item[].func()} aka simple macros), something like that should pop up:

This will save tons of time users currently have to spend browsing documentation about built-in macros, copy-pasting user macros to avoid mistyping them or painstakingly troubleshooting syntax issues with macro contexts, macro functions or "simple" macros.