no-unused-keys is very slow

[allanlewis]

What rule do you want to change?

@intlify/vue-i18n/no-unused-keys

Does this change cause the rule to produce more or fewer warnings?

No difference.

How will the change be implemented? (New option, new default behavior, etc.)?

Unclear.

Please provide some example code that this change will affect:

As per rule docs.

What does the rule currently do for this code?

Reports correctly, but slowly.

What will the rule do after it's changed?

The same, but more quickly.

Additional context

I used TIMING=1 to profile my ESLint config, which includes the following rule config:

  '@intlify/vue-i18n/no-unused-keys':
    - error
    - extensions: [.js, .json, .vue]

@intlify/vue-i18n/no-unused-keys came out as taking almost half the overall runtime of ESLint! I found that setting the src option reduced the runtime but it's still the slowest rule in my config by a significant margin.