Check constants passed to arguments of fromJSON calls are valid as JSON

[rhysd]

I noticed that it is popular to pass a string constant to fromJSON() to create an immediate array/object value. Here is a simple example:

on: push

jobs:
  test:
    runs-on: ubuntu-latest
    if: ${{ contains(fromJSON('["refs/heads/main", "refs/heads/master"]'), github.ref) }}
    steps:
      - run: echo hello

It would be nice to check the string literals are valid as JSON.

[rhysd]

It is also possible to strongly type the return value of fromJSON but I'm not sure this is really needed.