Plugins: Selector support for visitors

Writing down some idea(s):

> https://eslint.org/docs/developer-guide/selectors

```js
{
"IfStatement > BlockStatement": function(blockStatementNode) {
// ...your logic here
},
}
```
```
"no-restricted-syntax": ["error", "IfStatement > :not(BlockStatement).consequent"]
```

Not sure if a lot of people use this, but it could be convenient for simpler transforms. ESLint uses https://github.com/estools/esquery but not sure if we can just use it or we need to change something based on the AST (ideally it would be generic enough). (also found https://github.com/phenomnomnominal/tsquery)

We sorta have something you can do with "virtual ast nodes"/"aliases" but not sure how that works.

https://github.com/babel/babel/blob/a3f00896f710a95ed38f2f9fb3a6e048148b0b98/packages/babel-traverse/src/path/lib/virtual-types.js#L80-L84

```js
export const BlockScoped = {
checkPath(path: NodePath): boolean {
return t.isBlockScoped(path.node);
},
};
```

Was just sorta thinking if you wanted to target an IIFE for example (give the thing a name, it's not a normal AST node), you could do this (assuming you defined what an IIFE was somewhere):

```js
visitor: {
IIFE(path) {
// code here
}
}
```

Could certainly add this as a feature but I would just implement this as a babel plugin for people to try out before actually adding it in? Just seems better to do it this way. Would be something like this for the input/output:

```diff
visitor: {
- "FunctionDeclaration[params.length>3]"(path) {
- // code
- },
+ FunctionDeclaration(path) {
+ if (path.node.params.length > 3) {
+ // code
+ }
}
}
```

I see there's also https://github.com/dfilatov/jspath