feat: set up concurrency on building the lambdas

[JoaquinFernandez]

What has been implemented?

For some use cases, where there is limited system memory available (i.e. GitHub Actions 7GB), esbuild can run out of memory and crash when building a big serverless project (i.e. 50+ lambdas).

In order to overcome that I've added the esbuild option concurrency to only allow as much as concurrency promises running at the same time when building the lambdas.

I could help with the tests, and indeed I tried to work on them, but there is a lot to reestructure in the index.ts before unit tests start to make sense.

Closes #200

Steps to verify

• Run serverless package command with the esbuild option concurrency as an integer

  • It should limit the concurrent building of the service lambda's to that amount

• Run serverless package command with the esbuild option concurrency as a string

  • It should throw an error with the message:
    • TypeError: Expected concurrency to be an integer from 1 and up or Infinity, got a (string)

• Run serverless package command with no esbuild option concurrency

  • It should concurrently build all of the service lambda's

[github-actions]

🎉 This PR is included in version 1.19.0 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

Your semantic-release bot 📦🚀