Sarpik/get input list support (#829)

* feat(core): Create `getInputList` utility

Signed-off-by: Kipras Melnikovas <kipras@kipras.org>

* chore(core): Document usage of '\n' instead of [] @ `getInputList`

Signed-off-by: Kipras Melnikovas <kipras@kipras.org>

* test(core): Create a very simple test for `getInputList`

Signed-off-by: Kipras Melnikovas <kipras@kipras.org>

* run linter

* update commands/readme

Co-authored-by: Kipras Melnikovas <kipras@kipras.org>

README.md

@@ -23,6 +23,7 @@ Outputs can be set with `setOutput` which makes them available to be mapped into
 ```js
 const myInput = core.getInput('inputName', { required: true });
 const myBooleanInput = core.getBooleanInput('booleanInputName', { required: true });
+const myMultilineInput = core.getMultiline('multilineInputName', { required: true });
 core.setOutput('outputKey', 'outputVal');
 ```