no-unstable-nested-components does not detect components starting with underscore

[WickyNilliams]

A pattern we use commonly in our codebase (pre-dates me, and we're too far along the road now to change) is to prefix some components with an underscore e.g. when forwarding refs, or to signal a component shouldn't be exported etc.

Components with this naming convention do not get detected as parent components, and thus the unstable nested components are not flagged

function _Parent() {
  function NotFlagged() {
    return <b>oh no</b>
  }

  return <NotFlagged />
}

Given there have been other issues raised relating, and carve outs added for, this naming convention:

• [Bug]: react/prop-types not working when prefixing function with underscore _ #3560
• jsx-pascal-case: Allow leading underscore #2603

I think this constitutes a bug/oversight in this rule. Happy to make a PR, if there is agreement

[ljharb]

While I think a codemod would be trivial to change this pattern, I agree that since React supports a leading underscore, so too must we.