fix(treesitter): use subpriorities for tree ordering

This partially reverts 0b8a72b739,
that is unreverts 15e77a56b7

"priority" is an internal neovim concept which does not occur in shared
queries. Ideally a single priority space should eventually be enough
for our needs. But as we don't want to poke at the usages of
priorities right now in the wider ecosystem,
introduce the "subpriorities" so that treesitter code can distinguish
highlights of the same priorities with different tree nesting depth.

This mainly affects `injection.combined` as parent-tree nodes might appear
in the middle of child-tree nodes which otherwise is not possible.

runtime/lua/vim/_meta/api_keysets.lua

@@ -423,6 +423,7 @@ error('Cannot require a meta file')
 --- @field undo_restore? boolean
 --- @field url? string
 --- @field scoped? boolean
+--- @field _subpriority? integer
 
 --- @class vim.api.keyset.user_command
 --- @field addr? any