Proper escape sequence intermediate handling

[aymanbagabas]

Many escape sequences ignore intermediate bytes. For example, Erase Characters (ECH) CSI <n> X doesn't have any intermediate bytes, and something like CSI ? X should be ignored. Instead, Ghostty treat CSI ? X as if it was CSI X and erases a character at the current cursor position.

printf 'ABC\b\x1b[?X'

Current output:

AB_

The cursor is at the 3rd column

Expected output (invalid sequence is ignored):

ABC

The cursor is at the 4th column

[mitchellh]

Ah yes that's right. This is some old stuff (in the Ghostty codebase). A lot of the original stream processing stuff ignored intermediates because we didn't handle them then! We should go back over and audit all the sequences that need this check and add it.

While doing so, we should ensure to add unit test coverage which is now available at the bottom of that file.