runtime(openPlugin): add <Plug>-mappings

runtime(openPlugin): add <Plug>-mappings

vim9script <scriptcmd> mappings relying on imports cannot be evaluated
outside of the script file with the imports, so do not work with plugins
like vim-which-key, which applies the mappings using feedkeys().

Using <Plug> mappings is one way to address this, and has the added
benefit of reading like a description for users finding the mappings.

related: #17563

Signed-off-by: Mark Woods <mwoods.online.ie@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>