String Additions

String capitalization. Index-based insertion, replacement, and removal. Similar implementation of C# String.Format(..) to allow for terse index-based string interpolation without needing to use a literal with '$var' at the time of interpolation.
Consider adding the following tests:
test('should correctly handle multiple spaces between words', () {
  expect(capitalize('this was  a   huge    success.'), 'This Was  A   Huge    Success.');
});
test('should correctly handle a variety of word breaks', () {
  expect(capitalize('this\twas\na\rhuge\vsuccess.'), 'This\tWas\nA\rHuge\vSuccess.');
});
test('should modify only the first character of each word', () {
  expect(captalize('tHIS WAS a tRiUMpH.'), 'THIS WAS A TRiUMpH.');
});
test('should leave non-capitlizable characters unmodified', () {
  expect(capitalize('大成功 でした。'), '大成功 でした。');
});