Support for ESC X character strings

[EnronEvolved]

Is your feature request related to a problem? Please describe.
ECMA-48 section 8.3.128 defines the control function "start of string" (SOS) to initiate a "character string" terminated by a string terminator, in a similar (but not strictly identical) manner to an OSC, DCS, PM, or APC sequence.

Kitty currently does not currently recognise SOS sequences. Character strings initiated by them are printed to the terminal instead of forming an escape sequence.

Several other terminal emulators (Gnome Terminal, Terminator, xterm, Alacritty) implement this escape sequence (in a looser sense) without — as far as I know — doing anything with them.

Describe the solution you'd like
Implementation of SOS sequences, either in the strict sense defined in ECMA-48 or the looser DEC sense of the aforementioned emulators.

Describe alternatives you've considered
There aren't really any alternatives: either it gets implemented or it doesn't.