feat: add terminalcursor option

[CatsDeservePets]

This PR adds the option terminalcursor.
It allows changing the appearance (i.e. shape and blinking) of the cursor and accepts one of these values: default, block, underline, bar, blinkblock, blinkunderline, blinkbar.
By default, tcell uses default anyway, so we only have to call SetCursorStyle when actually overwriting it.

Why not just change the default settings for your terminal?
In all my shells, I am using vi modes which have their own cursor settings that take precedence over the terminal emulator config. That means I am not seeing the default cursor anyway most of the time.
Doing it like this tty-write "\033[5 q" does not work either.

Things to consider:

• SetCursorStyle also accepts optional colour parameters. If this should be supported, it might be better to change it from string to []string and perhaps also make blinking its own attribute.

• I chose the name terminalcursor over cursorstyle to avoid confusion with cursoractivefmt, cursorparentfmt and cursorpreviewfmt. However, I am open for naming suggestions, including the styles themselves.

• After executing shell-wait commands and displaying waitmsg, the default cursor is shown. This is caused by calling Suspend before, which temporarily restores terminal settings.

[CatsDeservePets]

@joelim-work Any thoughts on this? If you think this is too niche of a feature, I am fine with not merging it.

[joelim-work]

Thanks, I think this feature is fine to add, as other programs like Vim also allow you to change the cursor shape.

[joelim-work]

LGTM, thanks once again!