feat: export xterm types

[unional]

fixes #25

Note that the syntax export types * needs TS 5

[PabloLION]

Hi @unional . I made a new similar repo on https://github.com/PabloLION/xterm-react and I'm trying to improve it now. This export helps user to not import those interfaces and types from xtermjs to keep code cleaner and more robust (in case if xtermjs updates).
Do you want to add a similar PR to get some credits? (since it's only one line I'll do it my self at some point)