Support short flags

[karstenpedersen]

Add support for short flags

Fixes #58

• Add short: Option(String) field on Flag
• Add shorthands lookup dictionary on Flags that maps short flags to regular flags
• Add flag_short function to add shorthands to flags

---

I am a bit unsure what the best way is to make sure that short flags are exactly one character. Right now I just ignore them like this if they are invalid:

pub fn flag_short(for flag: Flag(a), of short: String) -> Flag(a) {
  case string.length(short) {
    1 -> Flag(..flag, short: Some(short))
    _ -> flag
  }
}

It could maybe be good to check it when the CLI execute.

examples/hello

Here is the repeat flag from the hello example:

pub fn repeat_flag() -> glint.Flag(Int) {
  use n <- glint.flag_constraint(
    glint.int_flag("repeat")
    |> glint.flag_short("r") // <---- NEW FUNCTION
    |> glint.flag_default(1)
    |> glint.flag_help("Repeat the message n-times"),
  )
  case n {
    _ if n > 0 -> Ok(n)
    _ -> snag.error("Value must be greater than 0.")
  }
}

This is the output we get when we run it with -h:

gleam run -m examples/hello -- -h
   Compiled in 0.03s
    Running examples/hello.main
It's time to say hello!

Prints Hello, <names>!

USAGE:
    gleam run -m examples/hello ( single ) [ 1 or more arguments ] [
        --caps=<BOOL> --repeat=<INT> ]

FLAGS:
        --caps=<BOOL>     Capitalize the hello message
    -h, --help            Print help information
    -r, --repeat=<INT>    Repeat the message n-times

SUBCOMMANDS:
    single                Prints Hello, <name>!

This is what we get when we run with -r=3:

gleam run -m examples/hello -- World -r=3
   Compiled in 0.04s
    Running examples/hello.main
Hello, World!
Hello, World!
Hello, World!