feat(core): add getYamlInput function

[qoomon]

Use Case Example

 - uses: foo/bar@v1
    with:
        permissions: |
             foo: admin
             bar: write

[rindeal]

So, you want to add an additional dependency to a core package just so that you can type:

core.getYamlInput(input)

instead of

YAML.parse(core.getInput(input))

Yeah, that's not going to happen, mate.

[qoomon]

@rindeal I think you got a point :-D