feat(mix): add depType support

[sheerlox]

Changes

Adds support for depType based on the only Mix dependency option.

Context

I'm using Conventional Commits & Semantic Release for my Elixir projects. Not having depType available means I have to choose between always releasing (build(deps):) or never releasing (chore(deps):) dependency updates.

Documentation (please check one with an [x])

• I have updated the documentation, or
• No documentation update is required

How I've tested my work (please select one)

I have verified these changes via:

• Code inspection only, or
• Newly added/modified unit tests, or
• No unit tests but ran on a real repository, or
• Both unit tests + ran on a real repository

[rarkins]

It seems you're reusing the npm terms "dependencies" and "devDependencies" but that's not necessary. Probably more intuitive for mix users if they are like "prod" and "dev"?