When a plugin registers a TermRequest handler there is currently no way
for the handler to know where the terminal's cursor position was when
the sequence was received. This is often useful information, e.g. for
OSC 133 sequences which are used to annotate shell prompts.

Modify the event data for the TermRequest autocommand to be a table
instead of just a string. The "sequence" field of the table contains the
sequence string and the "cursor" field contains the cursor
position when the sequence was received.

To maintain consistency between TermRequest and TermResponse (and to
future proof the latter), TermResponse's event data is also updated to
be a table with a "sequence" field.

BREAKING CHANGE: event data for TermRequest and TermResponse is now a
table

The data to be written can be very long, so use nvim_chan_send() instead
of io.stdout:write() as the latter doesn't handle EAGAIN.

A difference of these two approaches is that nvim_chan_send() writes to
stderr, not stdout, so it won't work if client stderr is redirected.

Co-authored-by: Gregory Anders <greg@gpanders.com>

Use the XTGETTCAP sequence to determine if the host terminal supports
the OSC 52 sequence and, if it does, enable the OSC 52 clipboard
provider by default.

This is only done automatically when all of the following are true:

  1. Nvim is running in the TUI
  2. 'clipboard' is not set to unnamed or unnamedplus
  3. g:clipboard is unset
  4. Nvim is running in an SSH connection ($SSH_TTY is set)
  5. Nvim is not running inside tmux ($TMUX is unset)

Problem:
Platform-specific UI providers should live in `vim.ui.*`. #24164

Solution:
- Move `vim.clipboard.osc52` module to `vim.ui.clipboard.osc52`.
- TODO: move all of `clipboard.vim` to `vim.ui.clipboard`.

ref #25872