Move away from using enums for Deno.Signal API

[bartlomieju]

Currently we got two huge enums for Signal definition:

deno/cli/dts/lib.deno.unstable.d.ts

Lines 567 to 637 in 77ead8a

	enum LinuxSignal {
	SIGHUP = 1,
	SIGINT = 2,
	SIGQUIT = 3,
	SIGILL = 4,
	SIGTRAP = 5,
	SIGABRT = 6,
	SIGBUS = 7,
	SIGFPE = 8,
	SIGKILL = 9,
	SIGUSR1 = 10,
	SIGSEGV = 11,
	SIGUSR2 = 12,
	SIGPIPE = 13,
	SIGALRM = 14,
	SIGTERM = 15,
	SIGSTKFLT = 16,
	SIGCHLD = 17,
	SIGCONT = 18,
	SIGSTOP = 19,
	SIGTSTP = 20,
	SIGTTIN = 21,
	SIGTTOU = 22,
	SIGURG = 23,
	SIGXCPU = 24,
	SIGXFSZ = 25,
	SIGVTALRM = 26,
	SIGPROF = 27,
	SIGWINCH = 28,
	SIGIO = 29,
	SIGPWR = 30,
	SIGSYS = 31,
	}
	enum MacOSSignal {
	SIGHUP = 1,
	SIGINT = 2,
	SIGQUIT = 3,
	SIGILL = 4,
	SIGTRAP = 5,
	SIGABRT = 6,
	SIGEMT = 7,
	SIGFPE = 8,
	SIGKILL = 9,
	SIGBUS = 10,
	SIGSEGV = 11,
	SIGSYS = 12,
	SIGPIPE = 13,
	SIGALRM = 14,
	SIGTERM = 15,
	SIGURG = 16,
	SIGSTOP = 17,
	SIGTSTP = 18,
	SIGCONT = 19,
	SIGCHLD = 20,
	SIGTTIN = 21,
	SIGTTOU = 22,
	SIGIO = 23,
	SIGXCPU = 24,
	SIGXFSZ = 25,
	SIGVTALRM = 26,
	SIGPROF = 27,
	SIGWINCH = 28,
	SIGINFO = 29,
	SIGUSR1 = 30,
	SIGUSR2 = 31,
	}
	/** **UNSTABLE**: Further changes required to make platform independent.
	*
	* Signals numbers. This is platform dependent. */
	export const Signal: typeof MacOSSignal | typeof LinuxSignal;

This is bad, as there are different enums depending on the OS.

After initial discussion between the core team we decided we should just use strings for signals.

This is prerequisite for #11757