Make `allow` and `ignore` symmetric

Currently, Dependabot offers two ways of opting dependencies in or out of updates: allow and ignore. The main differences (from a config file functionality standpoint) are:

• allow has the ability to specify dependency-type's (e.g. development, production) to allow a certain number of dependencies
• ignore has the ability to opt out of certain versions, while allow can't opt you in to certain versions (e.g. only allow updates on x.y.z not x+1.y.z).

It certainly feels useful to be able to write something like:

updates:
  - package-ecosystem: "npm"
    directory: "/"
    schedule:
      interval: "daily"
    ignore:
      - dependency-type: "development"

Which would fulfill issues like https://github.com/dependabot/dependabot-core/issues/3475

We also offer a @dependabot ignore command without a corresponding @dependabot allow command, though dependabot updates everything by default, so allow might be redundant, unless you're trying to opt back in after you've opted out.

As an internal implementation note, I believe allow and ignore run at different times, so potentially we might want to unify this, or further clarify this behavior.

No implementation files or tests are named. Start by tracing how the configuration handles allow and ignore, including their stated processing order, then review the @dependabot ignore command and issue 3475. Done means the agreed allow and ignore behavior is symmetric for dependency types, versions, and commands.