[compiler] Deprecate noEmit, add outputMode

This deprecates the `noEmit: boolean` flag and adds `outputMode: 'client' | 'client-no-memo' | 'ssr' | 'lint'` as the replacement. OutputMode defaults to null and takes precedence if specified, otherwise we use 'client' mode for noEmit=false and 'lint' mode for noEmit=true.

Key points:
* Retrying failed compilation switches from 'client' mode to 'client-no-memo'
* Validations are enabled behind Environment.proto.shouldEnableValidations, enabled for all modes except 'client-no-memo'. Similar for dropping manual memoization.
* OptimizeSSR is now gated by the outputMode==='ssr', not a feature flag
* Creation of reactive scopes, and related codegen logic, is now gated by outputMode==='client'