feat: Elixir support (#4496)

Dockerfile

docs/usage/configuration-options.md

lib/config/definitions.ts

lib/datasource/hex/index.ts

lib/manager/index.ts

lib/manager/mix/artifacts.ts

lib/manager/mix/extract.ts

lib/manager/mix/index.ts

lib/manager/mix/readme.md

lib/manager/mix/update.ts

lib/versioning/hex/index.ts

lib/versioning/hex/readme.md

renovate-schema.json

test/config/__snapshots__/validation.spec.ts.snap

test/datasource/hex.spec.ts

test/manager/mix/__snapshots__/artifacts.spec.ts.snap

test/manager/mix/__snapshots__/extract.spec.ts.snap

test/manager/mix/_fixtures/mix.exs

test/manager/mix/artifacts.spec.ts

test/manager/mix/extract.spec.ts