Add command to get terminal size

[noelwelsh]

Add command (effect, program, documentation) to get the current terminal size.

It looks like the size can be obtained with the command CSI18t, but it's not clear this is widely supported. If it's not widely supported, the next step is to use the ioctl C API.

See https://invisible-island.net/xterm/ctlseqs/ctlseqs.html and search for XTWINOPS for the escape code to get the terminal size (we're interested in getting it in characters; pixels doesn't seem as useful).

For example of C code using ioctl see https://rosettacode.org/wiki/Terminal_control/Dimensions#Library:_BSD_libc

Crossterm code, which is a bit more complicated, is at https://github.com/crossterm-rs/crossterm/blob/e104a7cb400910609cdde36f322e3905c4baa805/src/terminal/sys/unix.rs#L59

[kapunga]

It looks like the TIOCGWINSZ constant needed for use with ioctl is missing from the Scala native implementation, but JLine appears to have it: https://github.com/search?q=repo%3Ajline%2Fjline3%20TIOCGWINSZ&type=code

[noelwelsh]

The Scala Native people might be open to a PR, but it's probably easiest to start with the CSI code and see how widely supported that is.

[noelwelsh]

At the OSS Hack Night we discovered:

• Not all terminals respond to ESC[18t. The terminal inside IntelliJ did not but Ghostty did.
• JLine provides a method to get the size of the terminal, so that provides an implementation pathway on the JVM

As a step forward we can define a GetSize effect and implement it for the JVM.

[kapunga]

JLine provides a method to get the size of the terminal, so that provides an implementation pathway on the JVM
For a lot of things JLine is doing, it appears that eventually there is an endpoint in JNI we might be able to mimic when we worry about support for Scala Native.