'read_esc' will improperly interpret non SGR CSI as CSI under some circumstances

[brodieG]

Using sgr_to_html to highlight that it is getting interpreted:

> fansi::sgr_to_html("\033[31;20xhello\033[m")
[1] "<span style='color: #BB0000;'>hello</span>"
Warning message:
In fansi::sgr_to_html("\033[31;20xhello\033[m") :
  Encountered a non-SGR CSI sequence, see `?unhandled_ctl`; you can use `warn=FALSE` to turn off these warnings.
> fansi::sgr_to_html("\033[31xhello\033[m")
[1] "hello"
Warning message:
In fansi::sgr_to_html("\033[31xhello\033[m") :
  Encountered a non-SGR CSI sequence, see `?unhandled_ctl`; you can use `warn=FALSE` to turn off these warnings.