Problem:
We want to encourage implementing core features in Lua instead of C, but
it's clumsy because:
- Core Lua code (built into `nvim` so it is available even if VIMRUNTIME
  is missing/invalid) requires manually updating CMakeLists.txt, or
  stuffing it into `_editor.lua`.
- Core Lua modules are not organized similar to C modules, `_editor.lua`
  is getting too big.

Solution:
- Introduce `_core/` where core Lua code can live. All Lua modules added
  there will automatically be included as bytecode in the `nvim` binary.
- Move these core modules into `_core/*`:
  ```
  _defaults.lua
  _editor.lua
  _options.lua
  _system.lua
  shared.lua
  ```

TODO:
- Move `_extui/ => _core/ui2/`

Problem:
Terminals should respond with the terminator (either BEL or ST) used in
the query so that clients can reliably parse the responses. The
`TermRequest` autocmd used to handle background color requests in the
terminal does not have access to the original sequence terminator, so it
always uses BEL. #37018

Solution:
Update vterm parsing to include the terminator type, then forward this
data into the emitted `TermRequest` events for OSC/DCS/APC sequences.
Update the foreground/background `TermRequest` callback to use the same
terminator as the original request.

Details:
I didn't add the terminator to the `TermResponse` event. However, I
assume the `TermResponse` event doesn't care about the terminator
because the sequence is already parsed. I also didn't update any of the
functions in `src/nvim/vterm/state.c` that write out responses. It
looked like those all pretty much used ST, and it would be a much larger
set of changes. In that same file, there's also logic for 8 bit ST
sequences, but from what I can tell, 8 bit doesn't really work (see `:h
xterm-8bit`), so I didn't use the 8 bit ST at all.

Problem:
The execution of startup scripts in parent directories are too late
compared to scripts in current direcctory.

Solution:
Execute all startup scripts with `lua/_core/exrc.lua`.

closes: #35147

We don't need to specify the timeout ms here anymore, because the
implementation was changed to use it by default.

Problem:
LSP incremental selection provides default visual-mode keymaps for `an`
and `in`. Operator-pending mode is not supported, so `dan` and `can` do
not apply the operation.

Solution:
Modify selection_range() to be synchronous.
Add operator-pending mappings.

This also fixes inconsistent scrolling behavior on terminal output when
cursor is in the middle of the buffer and the scrollback is full.

Problem:  Wrong row in TermRequest with full scrollback.
Solution: Subtract by the number of lines deleted from scrollback.

Close #34938
Close #35030
Close #35233
Close #35259
Close #35290
Close #35433
Close #35541
Close #35766
Close #35792

Co-authored-by: huylg <45591413+huylg@users.noreply.github.com>
Co-authored-by: Jason Del Ponte <961963+jasdel@users.noreply.github.com>
Co-authored-by: sooriya <74165167+thuvasooriya@users.noreply.github.com>
Co-authored-by: Andrew Braxton <andrewcbraxton@gmail.com>
Co-authored-by: Enric Calabuig <enric.calabuig@gmail.com>
Co-authored-by: Augusto César Dias <augusto.c.dias@gmail.com>
Co-authored-by: David Sierra DiazGranados <davidsierradz@gmail.com>
Co-authored-by: Stepan Nikitin <90522882+vectravox@users.noreply.github.com>
Co-authored-by: Emilien Breton <bricktech2000@gmail.com>

This function allows the Nvim core to write arbitrary data to a TTY
connected to a UI's stdout.