String.Substring helper

[Andras-Csanyi]

This PR brings the following changes:

• Substring examples to the ConsoleApp
• Substring helper
• Substring tests

Notes:

• I made the changes on Mac, so .NET Framework tests and .NET Core 3.1 tests were not run, however I see that the CI build is executed on Windows Server. We will see if I introduced any suprise or not
• to avoid code duplication I treated the end parameter as default and it introduces a quirky corner case below
  • Substring "foobar" 0 0 will result foobar due to default(int) == 0 so, the code thinks end parameter were not be provided. From usage point of view I don't know when I would need a Substring "foobar" 0 0. I'm open for suggestions. The case is provided by a test case.
• there is another interesting corner case Substring "" 0 works, string.Substring() allows it. This is the reason only null input is checked by Guard and empty has an additional check.
• tests are provided for the exception paths too

Question:

• I see that test coverage can be increased by addtional test cases covering exception paths, do you see value in these? I'm happy to provide them as my time allows.
• I see that the the line endings are mixed LF and CRLF, it is intentional?
• Shall I extend the documentation with the new feature once it is approved and merged?

I'm looking forward to reading your feedback!

[codecov[bot]]

Codecov Report

Merging #76 (793725e) into master (11cf645) will increase coverage by 0.20%. The diff coverage is 100.00%.

:exclamation: Current head 793725e differs from pull request most recent head 24740a0. Consider uploading reports for the commit 24740a0 to get more accurate results

@@            Coverage Diff             @@
##           master      #76      +/-   ##
==========================================
+ Coverage   44.80%   45.01%   +0.20%     
==========================================
  Files          41       41              
  Lines        2649     2659      +10     
  Branches      364      366       +2     
==========================================
+ Hits         1187     1197      +10     
  Misses       1359     1359              
  Partials      103      103              

Impacted Files	Coverage Δ
...rc/Handlebars.Net.Helpers/Helpers/StringHelpers.cs	78.48% <100.00%> (+0.94%)	:arrow_up:

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[StefH]

CI Build is fine.

[StefH]

1. I see that test coverage can be increased by addtional test cases covering exception paths, do you see value in these? I'm happy to provide them as my time allows.
2. I see that the the line endings are mixed LF and CRLF, it is intentional?
3. Shall I extend the documentation with the new feature once it is approved and merged?

1--> Not really required.

2-->Not intentional, just an error I think...

3-->I'm not sure you can update the WIKI directly. But you can copy-paste the updates maybe here?