Add configuration for ELP language server (backport #11250)

[mergify]

Proposed Changes

Add a basic config file for the ELP language server so that language features such as code completion and go-to-definition work for ELP users.

Since RabbitMQ does not use either rebar3 or Buck2 as a build system, this configuration file is required to inform the language server about the basic structure of the project.

Types of Changes

What types of changes does your code introduce to this project?

• Bug fix (non-breaking change which fixes issue #NNNN)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause an observable behavior change in existing systems)
• Documentation improvements (corrections, new content, etc)
• Cosmetic change (whitespace, formatting, etc)
• Build system and/or CI

Checklist

• I have read the CONTRIBUTING.md document
• I have signed the CA (see https://cla.pivotal.io/sign/rabbitmq)

Not applicable:

• I have added tests that prove my fix is effective or that my feature works
• All tests pass locally with my changes
• If relevant, I have added necessary documentation to https://github.com/rabbitmq/rabbitmq-website
• If relevant, I have added this change to the first version(s) in release-notes that I expect to introduce it

---

This is an automatic backport of pull request #11250 done by [Mergify](https://mergify.com).