Razor.Extensions API polish

[NTaylorMullen]

General

• Make ViewComponentTagHelperMetadata static. - @NTaylorMullen
• PageDirective shouldn't double as a context object. It's highly inconsistent with the rest of the directives in the system. We should do a pass on it to make its public API surface look/feel like the other directives in Razor/Razor.Extensions. - @NTaylorMullen - Deemed unimportant.

[NTaylorMullen]

@rynowak @ajaybhargavb feel free to add your own items here.

[NTaylorMullen]

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