[naming-convention] modifier export - Allow selecting only exported members

[RebeccaStevens]

Repro

{
  "rules": {
    "@typescript-eslint/naming-convention": [
      "error",
      { "selector": "variable", "format": ["camelCase"] },
      // { "selector": "variable", "modifiers": ["export"], "format": ["UPPER_CASE"] }
    ]
  }
}

export const FOO = 'hello';
const bar = 'world';

Expected Result

The ability to enforce a naming convention that only applies to exported members.

Actual Result

Cannot narrow down selector enough.

Additional Info

Related to: #1612

Versions

package	version
@typescript-eslint/eslint-plugin	3.4.0
@typescript-eslint/parser	3.4.0
TypeScript	3.9.5
ESLint	7.2.0
node	14.4.0
npm	6.14.5

[bradzacher]

how come you want to specifically name exported variables differently to non-exported variables?

[RebeccaStevens]

When importing constants, I tend to like them being in upper case so that I can tell they are a constant at a glance and not something else like a function.

When just using constants in a module (without exporting them), there's no need to make upper case as it is clear from context what they are.

[Mossop]

I came to suggest the same thing but because I do things the other way around. I tend to use constants within a module for magic numbers and such but for exported objects prefer camel case.

[nicolas-cherel]

how come you want to specifically name exported variables differently to non-exported variables?

Looks to me that it is the reasons for public/private class members applies really well for exported and non exported identifiers within a module

[nicolas-cherel]

I'd also add this modifier for typeLike selector