fix(ui): re-organize tty fd handling and fix issues

- Use the correct fd to replace stdin on windows (CONIN)
- Don't start the TUI if there are no tty fd (not a regression,
  but makes sense regardless)
- De-mythologize "global input fd". it is just STDIN.

Also add the EXITFREE definition to main_lib rather than the nvim target, as the header generation needs the EXITFREE flag to work properly.

fixes #21610

- The defined interface for the UI is only the RPC protocol. The original
  UI interface as an array of function pointers fill no function.
- On the server, all the UI:s are all RPC channels.
  - ui.c is only used on the server.
  - The compositor is a preprocessing step for single-grid UI:s
- on the client, ui_client and tui talk directly to each other
  - we still do module separation, as ui_client.c could form the basis
    of a libnvim client module later.

Items for later PR:s
- vim.ui_attach is still an unhappy child, reconsider based on plugin experience.
- the flags in ui_events.in.h are still a mess. Can be simplified now.
- UX for remote attachment needs more work.
- startup for client can be simplified further (think of the millisecs we can save)

Problem:    When unibi_format() modifies params and data->buf overflows,
            unibi_format() is called again, causing the params to be
            modified twice. This can happen for escapes sequences that
            use the %i terminfo format specifier (e.g. cursor_address),
            which makes unibi_format() increase the param by 1.
Solution:   Make a copy of data->params before calling unibi_format().

It is fine to initialize ui_client_termname to NULL as it is only used
after tui_start().

feat(ui): refactor TUI from thread to separate process