Add grouping functionality and corresponding tests

Implemented a new function 'GroupCharsForMobile' in the 'apg' package, which groups characters in a mobile-friendly manner based on their Unicode category. Accompanying tests for this function have also been created in 'grouping_test.go'. This update enhances password readability on mobile devices.

grouping.go

@@ -0,0 +1,23 @@
+package apg
+
+import "unicode"
+
+// GroupCharsForMobile takes a given string of characters and groups them in a mobile-friendly
+// manner. The grouping is based on the following precedense: uppercase, lowercase, numbers
+// and special characters. The grouped string is then returned.
+func GroupCharsForMobile(chars string) string {
+	var uppers, lowers, numbers, others []rune
+	for _, char := range chars {
+		switch {
+		case unicode.IsUpper(char):
+			uppers = append(uppers, char)
+		case unicode.IsLower(char):
+			lowers = append(lowers, char)
+		case unicode.IsNumber(char):
+			numbers = append(numbers, char)
+		default:
+			others = append(others, char)
+		}
+	}
+	return string(uppers) + string(lowers) + string(numbers) + string(others)
+}

grouping_test.go

@@ -0,0 +1,24 @@
+package apg
+
+import "testing"
+
+func TestGroupCharacters(t *testing.T) {
+	tests := []struct {
+		name     string
+		password string
+		want     string
+	}{
+		{`PW: A1c9.Ba`, `A1c9.Ba`, `ABca19.`},
+		{`PW: PX4xDoiKrs,[egEAief{`, `PX4xDoiKrs,[egEAief{`, `PXDKEAxoirsegief4,[{`},
+		{`PW: *Z%C9d+PZYkD7D+{~r'w`, `*Z%C9d+PZYkD7D+{~r'w`, `ZCPZYDDdkrw97*%++{~'`},
+		{`PW: 4?r2YV:Abo&/z<3tJ*Z{`, `4?r2YV:Abo&/z<3tJ*Z{`, `YVAJZrbozt423?:&/<*{`},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			grouped := GroupCharsForMobile(tt.password)
+			if grouped != tt.want {
+				t.Errorf("GroupCharsForMobile() failed, expected: %s, got: %s", tt.want, grouped)
+			}
+		})
+	}
+}