Improve bit_set support in the flags package

[mtarik34b]

bit_set flags previously required a binary string (0s and 1s). This PR allows using the enum names directly, separated by commas. Binary string options can no longer start with an underscore.

Minimal example:

package named_bitset_flag

import "core:flags"
import "core:fmt"

E :: enum {
	Option_A,
	Option_B,
}

main :: proc() {
	Options :: struct {
		bitset: bit_set[E],
	}
	opt: Options

	args := [?]string {
		"-bitset:Option_B,Option_A"
	}
	flags.parse(&opt, args[:])

	fmt.println(opt.bitset)   // bit_set[E]{Option_A, Option_B}
}