feat(todos): Adds support for configuring todo decay days by rule

[scalvert]

Implements #1880 and supersedes #1964.

This change adds support for using a config file, .lint-todorc.js. This config is a superset of the old config, which was simply:

{
  "lintTodo": {
    "daysToDecay": {
      "warn": 5,
      "error": 10
    }
  }
}

The new structure adds a level of nesting, allowing you to specify which engine the config section applies to:

 {
   "lintTodo": {
     "ember-template-lint": {
       "daysToDecay": {
         "warn": 5,
         "error": 10
       },
       "daysToDecayByRule": {
         "no-bare-strings": { "warn": 10, "error": 20 }
       }
     }
   }
 }

In addition, you can now configure daysToDecay by ruleID, allowing more fine-grained configuration of decay dates. It also provides a single source of truth for configuring those dates.

As of this change, we now support

1. Legacy configs, such as those in the first example above
2. Configuration in package.json
3. Configuration in .lint-todorc.js

[MelSumner]

@scalvert confirmed that it throws the conflicting configuration file error if lint todo configurations exist in both package.json and .lint-todorc.js:

[scalvert]

@MelSumner thanks for checking that. That's not a very nice user experience. We can do better by adding a util in todo-utils that checks for this case, and allows consumers to validate early, and fail with a nicer formatted message. WDYT?

[rwjblue]

lint-todo/utils#242 lets us fix that issue (still needs tweaks here once its released to actually do it though).

[scalvert]

@MelSumner @rwjblue I updated the code to support validating the todo config beforehand, so it's cleaner output.

[scalvert]

@rwjblue any feedback on this one? It'd be nice to get it merged sooner rather than later.

[MelSumner]

Looking forward to this feature! 👍