feat: add typed flag names

[mfridman]

Fix #21.

This PR adds FlagName[T] so GetFlag can infer its return type:

const verbose cli.FlagName[bool] = "verbose"

f.Bool(string(verbose), false, "enable verbose output")

if s.GetFlag(verbose) { // bool inferred
      // ...
}

Existing string literals still work:

s.GetFlag[bool]("verbose")

Note, a string variable must first be converted to FlagName[T]. Supporting both directly would require two methods and I think the type-safety here and no additional method is worth the tradeoff. See the linked issue for more thoughts