Add Command Wrapper for getExecOutput

packages/exec/README.md

@@ -55,3 +55,57 @@ const exec = require('@actions/exec');
 
 await exec.exec('"/path/to/my-tool"', ['arg1']);
 ```
+
+#### CommandRunner
+
+CommandRunner is a more feature-rich alternative to `exec.getExecOutput`, it adds another level of abstraction to adjust behavior depending on execution results.
+
+Example with echo command on different platforms
+```js
+const commandRunner = exec.createCommandRunner()
+
+// Set command and arguments that are platform-specific
+if (IS_WINDOWS) {
+  runner.setCommand(await io.which('cmd', true))
+  runner.setArgs(['/c', 'echo'])
+} else {
+  runner.setCommand(await io.which('echo', true))
+}
+
+// Set arguments that should be added regardless of platform
+runner.setArgs((...args) => [...args, 'hello', 'world'])
+
+// Run just like exec.getExecOutput
+const { stdout, stderr, exitCode } = await runner.run()
+```
+
+Handling outputs example:
+```js
+await exec.createCommandRunner('echo', ['hello', 'world'])
+  .onExecutionError('fail', 'optional fail message') // will fail action if command failed to execute
+  .onStdError('log') // will log automatically generated message if command output has produced stderr
+  .onExitCode('> 0', 'throw') // will throw error on non-zero exit code
+  .run()
+```
+
+Handling options:  
+- `onEmptyOutput('throw' | 'fail' | 'log' | handler, [, message])` - triggers on empty output
+- `onExecutionError('throw' | 'fail' | 'log' | handler, [, message])` - triggers when failed to execute command itself
+- `onStdError('throw' | 'fail' | 'log' | handler, [, message])` - triggers when command reports that it was executed with errors
+- `onError('throw' | 'fail' | 'log' | handler, [,message])` - triggers either when failed to execute command or when command has been executed with stderr
+- `onSuccess('throw' | 'fail' | 'log' | handler, [, message])` - triggers when there's no errors and exitCode equals to zero
+- `onSpecificError(RegExp | string | matcherFn, 'throw' | 'fail' | 'log' | handler, [, message])` - matches specific error to handle
+- `onOutput(RegExp | string | matcherFn, 'throw' | 'fail' | 'log' | handler, [, message])` - matches specific stdout to handle
+- `onExitCode(string | number, 'throw' | 'fail' | 'log' | handler, [, message])` - matches specific exitCode to handle, when exitCode is passed is string it can be prefixed with an operator, i.e: `> 0`, `>= 1`, `= 31`, etc.
+- `
+on(eventOrEventArray,
+  'throw' | 'fail' | 'log' | handler
+  [, message]
+  )
+` - matches specific event in fasion similar to other handlers, but can be set to be triggered by different events passed as array (i.e. `['execerr', 'stderr'] - any error`) or by events not occuring using `!` prefix (i.e. `'!stdout' - empty output`), allowed events:
+  - 'execerr' - command failed to run
+  - 'stderr' - command had run but produced stderr
+  - 'stdout' - non-empty stdout
+  - 'exitcode' - non-zero exit code
+  - 'ok' - non-zero exit code + no stderr + no execerr
+