[breaking] Make Identifier an "atomic" node

`Identifier`s can have different properties totally unrelated to their "Identifierness":
- `optional` (e.g. in `function f(a?) {}` with Flow)
- `typeAnnotation` (e.g. in `var a: string;` with Flow/TypeScript)
- `decorators` (e.g. in `function f(@dec a) {}` with TypeScript)

Every other "atom" (e.g. strings, numbers, ...) don't have child nodes. Additionally, it makes it impossible to get the actual identifier name start/end position, because in `foo: string` `foo` isn't represented by anything. Example: [AST Explorer](https://astexplorer.net/#/gist/8ab88f26fe8714059fdc05d3387d5810/3247ca3be02eed96c7b617d91661656d1376ef6f).

Also, these properties don't make sense in many places: what does it mean if I add `typeAnnotation` to `prop` in `obj.prop`? Or `optional` to `label` in `break label;`?

I propose creating a new node, `AnnotatedPattern` (name suggestions are welcome), like this:

```js
defineType("AnnotatedPattern", {
builder: ["pattern", "typeAnnotation", "optional", "decorators"],
visitor: ["pattern", "typeAnnotation", "decorators"],
aliases: ["PatternLike", "LVal"],
fields: {
pattern: {
validate: assertNodeType("PatternLike"),
},
typeAnnotation: {
validate: assertNodeType("TypeAnnotation", "TSTypeAnnotation"),
optional: true,
},
optional: {
validate: assertValueType("boolean"), // Only valid if pattern is an identifier
optional: true,
},
decorators: {
validate: chain(
assertValueType("array"),
assertEach(assertNodeType("Decorator")),
),
optional: true,
},
},
});
```

`AnnotatedPattern` would be accepted in function parameters and in variable declarations. `typeAnnotation` & co would then be removed from destructuring patterns.

The issue names no repository files or tests. Start by locating Babel's AST node definitions and the handling of Identifier, patterns, function parameters, and variable declarations; compare the current child properties with the proposed AnnotatedPattern shape. Done means the AST structure and validations consistently make Identifier atomic without leaving the described properties on unrelated nodes.

Written by the indexing model from the issue text.