[StaticWebAssets] Adds incrementalism to DefineStaticWebAssets

[javiercn]

This shaves another ~600ms on builds

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[dotnet-policy-service]

Thanks for your PR, @javiercn.
To learn about the PR process and branching schedule of this repo, please take a look at the SDK PR Guide.

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[MackinnonBuck]

Looks great! I think I have too many comments right now for me to approve, but I'm not anticipating many changes will need to be made. Most comments are seeking clarification.

[MackinnonBuck]

Looks great!