Add new rules no-missing-message-ids and no-unused-message-ids

[bmish]

Fixes #57.

Detects:

• Using a messageId that does not exist
• Leaving a message in meta.messages that is unused

These were implemented as separate rules because:

• Each is quite complicated on its own and it helps to be able to test them in isolation
• In the event of false positives, the user can just disable one of the rules but not the other

Note that I eliminated many possible false positive scenarios by testing these rules on the ESLint codebase.

TODO

• Fix failing test case
• Write unit tests for util functions
• Add JSdoc for util functions
• Rename to require-valid-message-ids
• Split into separate rules no-missing-message-ids and no-unused-message-ids since the unused message ID check can have false positives when variables or helper functions are used
• Test on ESLint repository to detect edge cases
• Ignore usage check when can't statically determine messageIds used like below examples:

function reportEnd(node, messageId) {
    context.report({
        node,
        messageId
    });
}

• Consider variable messageId:

let messageId;
if (foo) {
  messageId = 'abc';
} else {
  messageId = 'def';
}

context.report({
  node,
  messageId
});

• Handle conditional in suggestions

context.report({
    node,
    messageId: "unexpectedRegExp",
    suggest: noFix ? [] : [{
        messageId: "replaceWithLiteral",
    }]
});

• Try to handle some false positives in eslint-plugin-unicorn with dynamic object keys:

const MESSAGE_ID = 'no-this-assignment';
const messages = {
	[MESSAGE_ID]: 'Do not assign `this` to `{{name}}`.',
};

After this is merged:

• Use util functions in Update no-missing-placeholders and no-unused-placeholders to handle messageIds #252

[bmish]

This is ready for review. These rules were fairly-challenging to implement given the number of false positives originally when I ran them on the ESLint codebase but I've solved all the issues.

[aladdin-add]

Looks so great! 💯

I've tested it in a few repos:

• eslint
• eslint-plugin-node
• eslint-plugin-react
• acorn (to find unexpected crashes on no-eslint-rule js)

It's working very well, thanks! 👍

[aladdin-add]

could the message be clearer, e.g. messageId "foo"?

[bmish]

Yes updated.

[bmish]

@aladdin-add updated the messages and I also fixed some false positives affecting eslint-plugin-unicorn due to variable message object keys.