fix(diagnostic): stack _tags hl-groups in a single extmark #38654

Problem:
Diagnostic highlight groups were applied by iterating and calling
`vim.hl.range` for each group individually. That resulted in multiple
extmarks with the same priority being created separately, which does not
allow `DiagnosticUnnecessary` and `DiagnosticDeprecated` with matching
options override `Diagnostic*` styling.

Solution:
Pass the list of hl-groups to `vim.hl.range` so they are applied
together in the correct order.

runtime/lua/vim/hl.lua

@@ -40,7 +40,8 @@ M.priorities = {
 ---
 ---@param buf integer Buffer number to apply highlighting to
 ---@param ns integer Namespace to add highlight to
----@param higroup string Highlight group to use for highlighting
+---@param higroup integer|integer[]|string|string[] Highlight group used for the text range.
+--- See the `hl_group` option in |nvim_buf_set_extmark()|.
 ---@param start [integer,integer]|string Start of region as a (line, column) tuple or string accepted by |getpos()|
 ---@param finish [integer,integer]|string End of region as a (line, column) tuple or string accepted by |getpos()|
 ---@param opts? vim.hl.range.Opts