Clarify handling of bigints in string power levels

**Link to problem area**: https://spec.matrix.org/v1.3/rooms/v1/#mroompower_levels-events-accept-values-as-strings

**Issue**

The spec currently doesn't put a limit on the range of integers allowed in string power levels. If this is taken to mean that there is no limit (which is what Synapse implemented, accidentally), this makes it rather complicated to support room versions 1 through 9 in homeservers other than Synapse. As of right now, Dendrite will reject events containing string power levels that exceed the limit of signed 64 bit integers, while Ruma will reject events containing string power levels that exceed the usual `MIN_SAFE_INTEGER` / `MAX_SAFE_INTEGER` limits imposed on non-stringified numbers in canonical JSON.

cc @neilalexander @turt2live @tulir

Start with the linked Matrix specification section on m.room.power_levels events and compare the stated rules with the Dendrite and Ruma limits described in the issue. Done means the specification explicitly defines the permitted range for string power levels and removes ambiguity for room versions 1 through 9.

Written by the indexing model from the issue text.