Support a directive to set a prefix that opts elements into Tag Helper processing (parse time)

[DamianEdwards]

We should support a new directive that allows the app to set a prefix that is used to opt elements into Tag Helper processing at parse time. It should default to nothing (no prefix). This is the functional opposite of the ! prefix for opting elements out of Tag Helper processing.

The directive should be hierarchical so that it can be set globally (in _GlobalImport.cshtml) or at any folder level.

It can be set to any string literal that the razor parser considers valid as part of an HTML element name.

.e.g. in _GlobalImport.cshtml

@tagHelperPrefix "th:"

in Views/Home/Index.cshtml

<input class="not-processed-for-tag-helpers" />
<th:input class="processed-for-tag-helpers" />
<cache>Not processed for tag helpers</cache>
<th:cache>Processed for tag helpers</th:cache>

[NTaylorMullen]

@DamianEdwards what do you think should happen in the case that a user has two of these on a single page? Error right?

[DamianEdwards]

@NTaylorMullen yep, error.

[NTaylorMullen]

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