Add IEnumerable implementation on String with exception throwing.

[flyingoverclouds]

Description

Adding a basic implementation of IEnumerable on String to avoid compiler crash when foreach-ing on a String.
The GetEnumator implementation only thow a NotSupportedException in order to be compliant with .net framework practices on NotImplemented/NotSupported/PlatformNotSupported exceptions.

Motivation and Context

Iterating with foreach on a string is a common practices in c#.
But the lake of IEnumerable implement in String class generate a compilation failure with a generic error message "CSC compiler error CS7038 'Failed to emit module'" , that is very confusing for a developper, source of lost time in debugging.

This correction remove the compiler CSC7038 error on compilation, and the developper will be quickly inform during debug that the GetEnumerator is not supported. It also add documentation and intellisense informations, improving quality of developper experience.

• Resolves CSC compiler error CS7038 "Failed to emit module" when using foreach() on a String Home#663.

Types of changes

• Improvement (non-breaking change that improves a feature, code or algorithm)
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Config and build (change in the configuration and build system, has no impact on code or features)
• Dependencies (update dependencies and changes associated, has no impact on code or features)

Checklist:

• My code follows the code style of this project.
• My change requires a change (update) to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[nfbot]

Hi @flyingoverclouds,
😞 I'm affraid you'll have to use the PR template like the rest of us...
Make sure you've used the template and have include all the required information and fill in the appropriate details. After doing that feel free to reopen the PR. If you have questions we are here to help..

[dnfadmin]

All CLA requirements met.

[josesimoes]

Great! Thanks for taking care of this.

(I've edited the PR message to comply with the template)