Clarify handling of bigints in string power levels

[jplatte]

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 exposed on non-stringified numbers in canonical JSON.

cc @neilalexander @turt2live @tulir

[deepbluev7]

See also #1099 (comment)

Originally it was intended to be part of the clarification, but got removed in the last second.