This moves our paste logic to `src/input` in preparation for exposing
this as part of libghostty-vt. This yields an immediate benefit of
unit tests for paste encoding.
Additionally, we were able to remove one allocation on every unbracketed
paste path unless the input specifically contains a newline. Unlikely to
be noticable, but nice.
NOTE: This also includes one change in behavior: we no longer encode
`\r\n` and a single `\r`, but as a duplicate `\r\r`. This matches xterm
behavior and I don't think will result in any issues since duplicate
carriage returns should do nothing in well-behaved terminals.
