Allow nested code fences

We may want to consider allowing nested code fences, so we can avoid code looking like this due to more feature flags being added over time:

```
///: BEGIN:ONLY_INCLUDE_IN(snaps)
export function removeSnap(
snapId: string,
): ThunkAction {
return async (
dispatch: MetaMaskReduxDispatch,
///: END:ONLY_INCLUDE_IN
///: BEGIN:ONLY_INCLUDE_IN(keyring-snaps)
getState,
///: END:ONLY_INCLUDE_IN
///: BEGIN:ONLY_INCLUDE_IN(snaps)
) => {
dispatch(showLoadingIndication());
///: END:ONLY_INCLUDE_IN
///: BEGIN:ONLY_INCLUDE_IN(keyring-snaps)
const subjects = getPermissionSubjects(getState()) as {
[k: string]: { permissions: Record };
};

const isAccountsSnap =
subjects[snapId]?.permissions?.snap_manageAccounts !== undefined;
///: END:ONLY_INCLUDE_IN

(...)
```

No file, parser, entry point, or test is named. Start by searching for handling of code fences and the ONLY_INCLUDE_IN markers, then determine the expected behavior for nested fences and add coverage showing that feature-flag filtering still produces valid code.

Written by the indexing model from the issue text.