Make @addTagHelper, @removeTagHelper and @tagHelperPrefix optionally accept quotes around their values

[NTaylorMullen]

We made a change to the addTagHelper, removeTagHelper and tagHelperPrefix directives to disallow quotes around their values. This happens to really mess with tooling when using rc1 with rc2+ bits. Not to mention, it doesn't hurt to have quotes. We should allow them optionally like we did in RC1.

/cc @DamianEdwards

[Eilon]

1. Let's re-add support for quotes, but make it some kind of warning.
2. Open a bug for RTM to remove all support for quotes.

[DamianEdwards]

@NTaylorMullen pls ensure you work with tooling to ensure their RC2 release supports both quotes and without (if anything special is required at all).

[mikes-gh]

@NTaylorMullen Yes I noticed VS complains without quotes with squiggly red line after these changes