Enable external extensions

[KhudaDad414]

Description:

1. Create a new command called 'persist' which is used to persist variables after an script completes running. this has been done to allow external extensions to define their own persist behaviour.
2. extract some useful methods from parallel command to utils.
3. Allow users to add/modify commands by defining custom extensions.

[codecov]

Codecov Report

Attention: Patch coverage is 73.91304% with 18 lines in your changes missing coverage. Please review.

Project coverage is 73.49%. Comparing base (b42f28f) to head (dac87be).

Files with missing lines	Patch %	Lines
lib/runner/run.js	27.77%	13 Missing ⚠️
lib/runner/util.js	89.65%	0 Missing and 3 partials ⚠️
lib/runner/index.js	60.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1503      +/-   ##
===========================================
- Coverage    73.68%   73.49%   -0.19%     
===========================================
  Files           45       46       +1     
  Lines         3420     3449      +29     
  Branches       986      993       +7     
===========================================
+ Hits          2520     2535      +15     
- Misses         701      715      +14     
  Partials       199      199              

Flag	Coverage Δ
integration	63.67% <73.91%> (-0.11%)	⬇️
legacy	39.22% <73.91%> (+0.10%)	⬆️
unit	39.28% <14.49%> (-0.22%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.