Rule Proposal: no-duplicate-keys-in-locale

[ota-meshi]

Please describe what the rule should do:

This rule reports duplicate keys within the same locale.

What category of rule is this? (place an "X" next to just one item)

[ ] Enforces code style (layout)
[X] Warns about a potential error (problem)
[ ] Suggests an alternate way of doing something (suggestion)
[ ] Other (please specify:)

Provide 2-3 code examples that this rule will warn about:

• en.for-module1.json

{
  "dupe-key1": "foo", // <- dupe-key1 are duplicates.
  "dupe-key1": "bar", // <- dupe-key1 are duplicates.
  "dupe-key2": "baz", // <- dupe-key2 duplicates the same locale key in another file.
}

• en.for-module2.json

{
  "dupe-key2": "qux", // <- dupe-key2 duplicates the same locale key in another file.
}

Additional context

I found in #79 that some users have multiple resources for the same locale.

I thought that if you split into multiple files, you will not be able to check for duplicate keys or missing keys, but I realized that this plugin mechanism can implement rules to check for duplicate keys and missing keys.

If we add this rule, we need to change the no-missing-keys rule to not report if you have at least one hit in the same locale.

I also considered whether a rule was needed to report keys that were existing in one locale and missing in another.
In my opinion, this rule is not needed as the reporting of the no-missing-keys rule meets this.

[kazupon]

We also need rules to detect duplicate messages as similar issues.
#8