Use global serverless executable if config option is set

[HyperBrain]

What did you implement:

Closes #1

How did you implement it:

If the serverless.useGlobal option is set to true, the extension will try to spawn a globally available serverless command.

How can we verify it:

Install serverless globally with npm install -g serverless and set the workspace option serverless.useGlobal: true. Then execute any serverless command via the context menu. The command should run and use the global serverless installation.

Todos:

• [ ] Write tests
• [ ] Write documentation
• [ ] Fix linting errors
• [ ] Make sure code coverage hasn't dropped
• [ ] Provide verification config / commands / resources
• [ ] Enable "Allow edits from maintainers" for this PR
• [ ] Update the messages below

Is this ready for review?: NO Is it a breaking change?: NO