chore: Added stricter linter rules

[albertodev01]

Why?

With the new Dart 2.14 release, there is a new recommended set of lints to apply. The package also needs stricter lint rules

What?

I have added the Dart's official lints package and the dart_code_metrics one as well

Types of Changes

• New feature (non-breaking change which adds functionality)

Notes

No new features added

Checklist

• I have provided a description of the proposed changes.
• I added unit tests for all relevant code.
• I added golden tests for all UI changes.
• I added documentation for all relevant code.

[codecov]

Codecov Report

Merging #11 (c1b0380) into master (c73279a) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master       #11   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            8         8           
  Lines          305       305           
=========================================
  Hits           305       305           

Impacted Files	Coverage Δ
lib/src/extensions/fraction_string.dart	100.00% <ø> (ø)
lib/src/extensions/mixed_string.dart	100.00% <ø> (ø)
lib/src/core.dart	100.00% <100.00%> (ø)
lib/src/egyptian.dart	100.00% <100.00%> (ø)
lib/src/mixed.dart	100.00% <100.00%> (ø)

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