feat(extmark): stack multiple highlight groups in hl_group

This has been possible in the "backend" for a while but
API was missing.

Followup: we will need a `details2=true` mode for `nvim_get_hl_id_by_name`
to return information in a way forward compatible with even further
enhancements.

runtime/lua/vim/_meta/api_keysets.lua

@@ -241,7 +241,7 @@ error('Cannot require a meta file')
 --- @field end_line? integer
 --- @field end_row? integer
 --- @field end_col? integer
---- @field hl_group? integer|string
+--- @field hl_group? any
 --- @field virt_text? any[]
 --- @field virt_text_pos? string
 --- @field virt_text_win_col? integer