Add 'getExtension' method to the File class

[chikaldirick]

This PR adds getExtension(): string method to the File.php class that by using pathinfo() method under the hood returns extension of a file as a result.
To confirm that everything works as expected I've attached a simple test in FileTest.php file that check few potential use cases.
This should close issue #556

[codecov]

Codecov Report

Merging #559 (33c7b9a) into master (ff88b13) will not change coverage.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##              master      #559   +/-   ##
===========================================
  Coverage     100.00%   100.00%           
- Complexity       972       973    +1     
===========================================
  Files            113       113           
  Lines           2405      2407    +2     
===========================================
+ Hits            2405      2407    +2     

Impacted Files	Coverage Δ
packages/framework/src/Models/Support/File.php	100.00% <100.00%> (ø)

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[caendesilva]

Looks fantastic, thank you for your contribution!