When using the tmux passthrough sequence any escape characters in the
inner sequence must be escaped by adding another escape character.

The OptionSet autocommand does not fire until Vim has finished starting,
so setting 'background' before the VimEnter event would not fire the
OptionSet event. The prior implementation also waited until VimEnter to
set 'background', so this was a regression introduced when moving
background detection into Lua.

Also update the type annotation of TSNode:id(), which returns a string,
not an integer.

Problem: Only injections under the top level tree are found.

Solution: Iterate through all trees to find injections. When two
injections are contained within the same node in the parent tree, prefer
the injection with the larger byte length.