Add exists() method to ScopedRef with tests

[jinyus]

Description

Re: #15

Test used to verify behaviour:

@yousefak007 lmk if it's missing anything.

final countRef = Ref.scoped((ctx) => 1);

await tester.pumpWidget(
        MaterialApp(
          home: LiteRefScope(
            child: Builder(
              builder: (context) {
                final initialized = countRef.exists(context);
                expect(initialized, false);

                final val = countRef.of(context);
                expect(val, 1);

                final initialized2 = countRef.exists(context);
                expect(initialized2, true);

                return LiteRefScope(
                  overrides: [
                    countRef.overrideWith((ctx) => 2),
                  ],
                  child: Builder(
                    builder: (context) {
                      final initialized = countRef.exists(context);
                      expect(initialized, false);

                      final val = countRef.of(context);
                      expect(val, 2);

                      final initialized2 = countRef.exists(context);
                      expect(initialized2, true);

                      return Text('$val');
                    },
                  ),
                );
              },
            ),
          ),
        ),
      );

Type of Change

• ✨ New feature (non-breaking change which adds functionality)
• 🛠️ Bug fix (non-breaking change which fixes an issue)
• ❌ Breaking change (fix or feature that would cause existing functionality to change)
• 🧹 Code refactor
• ✅ Build configuration change
• 📝 Documentation
• 🗑️ Chore

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (1485546) to head (7b43259).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main       #16   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            8         8           
  Lines          145       149    +4     
=========================================
+ Hits           145       149    +4     

Flag	Coverage Δ
unittests	100.00% <100.00%> (ø)

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[yousefak007]

Thanks, Yes this is good