feat(api): set nvim_echo() kind for ext_messages (#33998)

Problem:  Unable to emit a message with arbitrary kind.
Solution: Add a "kind" opts field to nvim_echo().
          Use it to set the "list_cmd" kind for vim.show_pos().

runtime/lua/vim/_meta/api_keysets.lua

@@ -90,6 +90,7 @@ error('Cannot require a meta file')
 --- @class vim.api.keyset.echo_opts
 --- @field err? boolean
 --- @field verbose? boolean
+--- @field kind? string
 
 --- @class vim.api.keyset.empty