feat: Added new 'MixedFraction.fromDouble' constructor

[albertodev01]

Why?

An issue (#18) pointed out the absence of a factory constructor.

What?

I have added the MixedFraction.fromDouble factory constructor to MixedFraction and tested it.

Types of Changes

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

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 #19 (bb25d3d) into master (05e92d9) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master       #19   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            9         9           
  Lines          322       325    +3     
=========================================
+ Hits           322       325    +3     

Impacted Files	Coverage Δ
lib/src/types/mixed.dart	100.00% <100.00%> (ø)

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