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/doc/api.txt

@@ -2660,6 +2660,8 @@ nvim_buf_set_extmark({buffer}, {ns_id}, {line}, {col}, {opts})
                     and below highlight groups can be supplied either as a
                     string or as an integer, the latter of which can be
                     obtained using |nvim_get_hl_id_by_name()|.
+                    Multiple highlight groups can be stacked by passing an
+                    array (highest priority last).
                   • hl_eol : when true, for a multiline highlight covering the
                     EOL of a line, continue the highlight for the rest of the
                     screen line (just like for diff and cursorline highlight).

runtime/doc/news.txt

@@ -188,6 +188,7 @@ API
 • |nvim_echo()| `err` field to print error messages and `chunks` accepts
   highlight group IDs.
 • |nvim_open_win()| `relative` field can be set to "laststatus" and "tabline".
+• |nvim_buf_set_extmark()| `hl_group` field can be an array of layered groups
 
 DEFAULTS
 

runtime/lua/vim/_meta/api.lua

@@ -595,6 +595,9 @@ function vim.api.nvim_buf_line_count(buffer) end
 --- - hl_group : highlight group used for the text range. This and below
 ---     highlight groups can be supplied either as a string or as an integer,
 ---     the latter of which can be obtained using `nvim_get_hl_id_by_name()`.
+---
+---     Multiple highlight groups can be stacked by passing an array (highest
+---     priority last).
 --- - hl_eol : when true, for a multiline highlight covering the
 ---            EOL of a line, continue the highlight for the rest
 ---            of the screen line (just like for diff and

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