[terminfo] add smxx/rmxx, purge blink

[dankamongmen]

Testing with WezTerm nightly indicates no blink support
available through the xterm \E[5m, and I didn't see any
support in the code -- purge it. smxx and rmxx, meanwhile,
have been added to indicate support for struck text.

[dankamongmen]

huh, i guess i need to edit termwiz/data/xterm-256color-italic.terminfo as well, don't i?

[wez]

#904 is adding blink support!

termwiz/data/xterm-256color-italic.terminfo is the termwiz crate's fallback for running on a modern xterm compatible system when we either can't find terminfo or are running on windows with virtual terminal support. It's also used by wezterm's format function, but that should really target wezterm.terminfo instead.

So: if xterm supports smxx and rmxx then it makes sense to add it to xterm-256color-italic.terminfo, but otherwise not!