Underscores in string powerlevels

**Link to problem area**: https://spec.matrix.org/v1.18/rooms/v9/#mroompower_levels-events-accept-values-as-strings

**Issue**
*Spawned from [discussion](https://matrix.to/#/!i5D5LLct_DYG-4hQprLzrxdbZ580U9UB6AEgFnk6rZQ/$3hp7bIvnhcdAI5pJdvvXKjDIptcD4WVDvi7toPuX1es?via=element.io&via=matrix.org&via=beeper.com) in the Synapse Development room*

Synapse would appear to allow underscores in string powerlevels as its only validation is passing the string into `int` and rejecting on a `ValueError`, but it is not raised:
```
>>> int("10_000_000")
10000000
>>>
```

This behaviour appears to have been added in Python 3.6: https://peps.python.org/pep-0515/

Given the transformations in that spec section are attemtping to mirror the functionality of `int`, should the spec be updated to reflect this, or Synapse?

Start with the linked Matrix specification section and compare its stated transformations with Python's int behavior, then review the linked Synapse Development discussion for the intended resolution. Done means deciding whether the specification or Synapse should change and documenting the resulting behavior in the relevant specification text.

Written by the indexing model from the issue text.