parser: Add Token() to Node interface

Add Token() to ast Node interface so that we can more accurately
pinpoint the error position, e.g.:

	func numf n:num
	    print n
	end

	numf "abc"

The example above only highlighted the end of line with the `numf` with
a message saying first argument is of wrong type, but now it also
highlights `"abc"` correctly.

This is still problematic for error sources that are whole nodes, e.g.:

	numf "abc"+"123"

Should probably highlight all of `"abc"+"123"` rather than just the
first token. However, the might end up as undesired result for _very_
long expressions or multiline array / map literals.

Author: juliaogris

pkg/evaluator/evaluator_test.go

@@ -789,7 +789,7 @@ func TestHasErr(t *testing.T) {
 	prog := `
 has ["a"] "a" // cannot run 'has' on array
 `
-	want := "line 2 column 15: 'has' takes 1st argument of type '{}', found '[]string'"
+	want := "line 2 column 5: 'has' takes 1st argument of type '{}', found '[]string'"
 	got := run(prog)
 	assert.Equal(t, want, got)
 }
@@ -825,7 +825,7 @@ func TestDelErr(t *testing.T) {
 	prog := `
 del ["a"] "a" // cannot delete from array
 `
-	want := "line 2 column 15: 'del' takes 1st argument of type '{}', found '[]string'"
+	want := "line 2 column 5: 'del' takes 1st argument of type '{}', found '[]string'"
 	got := run(prog)
 	assert.Equal(t, want, got)
 }