Add CSS selector for TagHelper required attributes.

[NTaylorMullen]

With the completion of aspnet/Razor#684 TagHelpers can now determine their application on an HTML element based on an attribute name and its value.

The following CSS selector attributes were added:

1. [name^=value] : HTML element must have an attribute with name (case insensitive) and a value that starts with value (case sensitive).
2. [name$=value] : HTML element must have an attribute with name (case insensitive) and a value that ends with value (case sensitive).
3. [name=value] : HTML element must have an attribute with name (case insensitive) and a value that equals value (case sensitive).

Full list here. We haven't added support for all of them, just the ones listed above.

Any attribute value with a dynamic value such as type="prefix @something suffix" would result in the following:

1. [type^=prefix] = Matches ✅
2. [type$=suffix] = Matches ✅
3. [type='prefix @something suffix'] = Does not match ❌

This is because dynamic values are treated as unmatchable pieces of an attribute value.

I've abstracted the logic for the selector matching criteria into a TagHelperDescriptors RequiredAttributes. Each RequiredAttribute is of type TagHelperRequiredAttributeDescriptor and has an IsMatch method which takes care of all of the criteria.

[sayedihashimi]

Assigned to @madskristensen

[sayedihashimi]

TFS Bug#231166

[webtools-bot]

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