Add option for concurrency of zipping packages

[fredrik]

Is your feature request related to a problem? Please describe.
For large projects with many functions, the packing stage can cause out-of-memory errors. This is due to the zipping of functions being done in parallel. (See https://github.com/floydspace/serverless-esbuild/blob/master/src/pack.ts#L174)

The problem is more likely to manifest with larger functions — the project I'm working ran into this issue when a new dependency caused the bundled function size to double.

Describe the solution you'd like
I'd like to limit the number of concurrent calls to zip in the package step using an optional configuration parameter.

Describe alternatives you've considered
Simply zipping one function (or a small pre-defined number of functions) at a time would also solve the problem, but might have other performance impacts.