feat: Add ESLINT_D_MAX_INSTANCES env variable. (#201)

[connorgmeehan]

Hey I left this branch on my local machine and figured I'd just push it. This resolves #201 by adding an environment variable to configure the max instances so it can be reduced/increased from 10.
It just needs some help getting the error logging working. I can't print to std out. Would you be able to help me with this?

[uloco]

Really looking forward to this. Any blocker that can be removed here?

[mantoni]

@uloco Thanks for offering help.

This will need test cases and the validation should happen when starting the server so that we can log to stderr or fail. Currently this happens inside of the daemon and the error isn't shown.

[uloco]

I'll see what I can do thx

[mantoni]

This is obsolete with the new major v14.0.0 (see #294). The new release spawns a separate instance per eslint installation. Please see the README on how to control the lifetime of each instance.