Problem:
`nvim_echo(…, {id=…})` accepts user-defined id as a string or integer.
Generated ids are always higher than last highest msg-id used. Thus
plugins may accidentally advance the integer id "address space", which,
at minimum, could lead to confusion when troubleshooting, or in the
worst case, could overflow or "exhaust" the id address space.
There's no use-case for it, and it could be the mildly confusing, so we
should just disallow it.
Solution:
Disallow *integer* user-defined message-id.
Only allow *string* user-defined message-id.
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