[VM/runtime] - detect null safety before isolate initialization

    - Detect null safety when not specified before isolate
      initialization
      - for source files by having CFE parse the source file
        for @dart annotations
      - for kernel files by sniffing the kernel file for
        compilation mode
      - for appJIT files by sniffing the feature string
      - for AOT snapshots by sniffing the feature string

    - Remove workaround of returning null safety to false during
      bootstrapping

    - Add a new Dart C API call for detecting null safety

Bug: 41766
Change-Id: Ia8cf264323a2d0d58c2855ce6491456aa6f1da07
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/150089
Commit-Queue: Siva Annamalai <asiva@google.com>
Reviewed-by: Alexander Markov <alexmarkov@google.com>