Extend DECSCUSR (Set Cursor Style) with hollow block option

[rockorager]

@dnkl is investigating extending the DECSCUSR enum to include blinking hollow block, and hollow block. Ghostty currently allows the configuration file to specify a hollow_block as the default cursor style, however an application cannot choose this explicitly via DECSCUSR. The idea is to extend the enum with

...
7 = blinking hollow block
8 = hollow block

Reference: foot pr adding the configuration option

[00-kat]

Cross-referencing the Kitty issue @rockorager opened about this; Kovid has some interesting arguments for (possibly) not supporting this there.

[mitchellh]

I don't have a strong opinion about this and I'm willing to follow the lead of other terminals.