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.

Problem:  No test for mode() when executing Ex commands
Solution: Add some test cases and simplify several other test cases.
          Also add a few more test cases for ModeChanged.

closes: vim/vim#13588

https://github.com/vim/vim/commit/fcaeb3d42b228e73c669b2fce78f1d3fe112769f

tmux intercepts and ignores XTGETTCAP so wrap the query in the tmux
passthrough sequence to make sure the query arrives at the "host"
terminal.

Users must still set the 'allow-passthrough' option in their tmux.conf.

feat(decoration): allow conceal_char to be a composing char