String typed TagHelper attributes don't provide instant IntelliSense for trailing periods

[NTaylorMullen]

For instance in file => new web application if you type the following you don't get "codification" of the trailing period, it's handled as markup until you type more (note the background color of the code):

This means if you try to invoke IntelliSense at the period location you don't get any. If you then type any additional code it then highlights:

This does not happen for non-TagHelper tags; they have a different functionality where the period is captured as code for a brief period of time and then transitioned into markup.

/cc @BrennanConroy

[NTaylorMullen]

Grabbing this since I'm low on issues to work on 😄. Feel free to point me towards something else more pressing if it exists.

[NTaylorMullen]

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