feat(string): add onlyDigits and isNumeric

lib/src/string.dart

@@ -113,6 +113,11 @@ extension StringIsLatin1Extension on String {
   }
 }
 
+extension StringOnlyDigitsExtension on String {
+  /// Returns a string containing only ASCII digit characters from this string.
+  String get onlyDigits => replaceAll(RegExp(r'[^0-9]'), '');
+}
+
 extension StringReversedExtension on String {
   /// Returns a new string with characters in reversed order.
   String get reversed {
@@ -160,6 +165,11 @@ extension StringIsDoubleExtension on String {
   bool get isDouble => toDoubleOrNull() != null;
 }
 
+extension StringIsNumericExtension on String {
+  /// Returns `true` if the string can be parsed as a number.
+  bool get isNumeric => toDoubleOrNull() != null;
+}
+
 extension StringToDoubleExtension on String {
   /// Parses the string as a [double] number and returns the result.
   double toDouble() => double.parse(this);

test/string_test.dart

@@ -31,6 +31,14 @@ void main() {
       expect('ő'.isLatin1, false);
     });
 
+    test('.onlyDigits', () {
+      expect(''.onlyDigits, '');
+      expect('abc'.onlyDigits, '');
+      expect('123'.onlyDigits, '123');
+      expect('Phone: +1 (555) 123-4567'.onlyDigits, '15551234567');
+      expect('a1 b2-c3'.onlyDigits, '123');
+    });
+
     test('.isBlank', () {
       expect('	 '.isBlank, true);
       expect('  .  '.isBlank, false);
@@ -132,6 +140,16 @@ void main() {
       expect('123456789.987654321'.isDouble, true);
     });
 
+    test('.isNumeric', () {
+      expect(''.isNumeric, false);
+      expect('a'.isNumeric, false);
+      expect('12abc'.isNumeric, false);
+      expect('1'.isNumeric, true);
+      expect('-42'.isNumeric, true);
+      expect('1.0'.isNumeric, true);
+      expect('123456789.987654321'.isNumeric, true);
+    });
+
     test('.toDouble()', () {
       expect('0'.toDouble(), 0.0);
       expect('0.0'.toDouble(), 0.0);