…serializers from single source (#14312)

Pure refactor. Makes only minor cosmetic changes to the generated raw transfer deserializer code, mainly just alters *how* that code is generated.

Previously we generated the source for the 2 different deserializers (JS and TS) separately, with the JS deserializer leaving out some fields e.g. `typeAnnotation`.

Instead, generate only 1 base implementation, with TS-only fields gated by `IS_TS`. `IS_TS` is defined as a `const` at top of the file.

```js
const IS_TS = true;

function deserializeFunction(pos) {
  const params = deserializeBoxFormalParameters(pos + 56);
  if (IS_TS) {
    const thisParam = deserializeOptionBoxTSThisParameter(pos + 48);
    if (thisParam !== null) params.unshift(thisParam);
  }

  return {
    type: deserializeFunctionType(pos + 84),
    params,
    ...(IS_TS && {
      declare: deserializeBool(pos + 87),
      typeParameters: deserializeOptionBoxTSTypeParameterDeclaration(pos + 40),
    }),
    // ... etc ...
  };
}
```

Then we generate both the JS and TS deserializers from this same file, by setting the `IS_TS` const to `true` or `false`, and using minifier to shake out the dead code.

This:

1. Simplifies custom deserializers.
2. Opens the door to add more `const` flags, e.g. `RANGES` to switch on/off including `range` field in AST nodes.