Allow fallback handlers at State layer for unrecognised CSIs/etc..

<tarruda>: Allow user callbacks to parse custom terminal codes such as bracketed paste or OCS 7: https://github.com/neovim/neovim/issues/2252

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This is likely to be implemented by taking an optional ParserCallback struct at the State and Screen layers, allowing a higher-level application to take control to implement other bits that libvterm itself doesn't like.

    void vterm_state_set_unrecognised_callbacks(VTermState *state, const VTermParserCallbacks *cb, void *user);
    void vterm_screen_set_unrecognised_callbacks(VTermScreen *screen, const VTermParserCallbacks *cb, void *user);