Making es-module-shims optional

[guybedford]

With better native import map support these days, we should improve the es-module-shims rules:

• We should only inject es-module-shims when using features that require polyfilling (eg Wasm modules / CSS modules etc)
• We should detect if es-module-shims is already used in an HTML injection, and take that preference as the existing preference

With the above, when injecting a standard import map workflow, es-module-shims shouldn't get pulled in by default.

It would also be good to have a flag to define these injection rules - --esms | --no-esms | --esms=auto as default kind of thing