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>

packages/core/__tests__/core.test.ts

@@ -29,6 +29,8 @@ const testEnvVars = {
   INPUT_WRONG_BOOLEAN_INPUT: 'wrong',
   INPUT_WITH_TRAILING_WHITESPACE: '  some val  ',
 
+  INPUT_MY_INPUT_LIST: 'val1\nval2\nval3',
+
   // Save inputs
   STATE_TEST_1: 'state_val',
 
@@ -166,6 +168,14 @@ describe('@actions/core', () => {
     )
   })
 
+  it('getMultilineInput works', () => {
+    expect(core.getMultilineInput('my input list')).toEqual([
+      'val1',
+      'val2',
+      'val3'
+    ])
+  })
+
   it('getInput trims whitespace by default', () => {
     expect(core.getInput('with trailing whitespace')).toBe('some val')
   })