More efficient isBlank check (probably)

[niktekusho]

Hi, thank you for the awesome library.

I noticed that isBlank is implemented as:

quiver-dart/lib/strings.dart

Line 19 in d9bf68e

bool isBlank(String? s) => s == null || s.trim().isEmpty;

I am new to Dart (learning it for Flutter) and expect that trim will create a new String to check if it is blank.
Is my assumption correct? I cannot confirm this in code since String is abstract, and I could not find an implementation of it (I think it's implemented like a native method in Java, though 😄 ).

Would you accept a PR that avoids the new string creation by checking if each code point of the original string is whitespace?

The implementation I have locally is the following:

/// Set with whitespace character points.
/// ```plaintext
///     0009..000D    ; White_Space # Cc   <control-0009>..<control-000D>
///     0020          ; White_Space # Zs   SPACE
///     [cut for brevity...] 
///     FEFF          ; BOM                ZERO WIDTH NO_BREAK SPACE
/// ```
const whitespaces = <int>{
  0x0009,
  0x000A,
  0x000B,
  0x000C,
  0x000D,
  0x0020,
  0x0085,
  0x00A0,
  0x1680,
  0x2000,
  0x2001,
  0x2002,
  0x2003,
  0x2004,
  0x2005,
  0x2006,
  0x2007,
  0x2008,
  0x2009,
  0x200A,
  0x2028,
  0x2029,
  0x202F,
  0x205F,
  0x3000,
  0xFEFF,
};

bool isBlank(String? val) {
  if (val == null) return true;

  return val.codeUnits.every((element) => whitespaces.contains(element));
}

Here are some incomplete tests (imports are from Flutter; thus, they would need a replacement from the tests package):

void main() {
  test('Passing null to isBlank returns true', () {
    expect(isBlank(null), isTrue);
  });

  test('Passing non empty string to isBlank returns false', () {
    expect(isBlank('abc'), isFalse);
  });

  test('Passing empty string to isBlank returns true', () {
    expect(isBlank(''), isTrue);
  });

  test('Passing blank string to isBlank returns true', () {
    expect(isBlank(' '), isTrue);
  });
}

It's surely a bit long and verbose, but I don't think it's hard to understand.
What do you think about it?
One problem that might arise in the future will be if new codepoints in Unicode become whitespaces, which will require a code change in this library.