Add a guppy v1 migration guide

[tatiana-s]

Main breaking change:

• Measurements are now lazy

Other:

• Mutable structs?

[mark-koch]

std.collections.{Stack, Queue, PriorityQueue} are now mutated in place instead of returning a new state. Also, their members are now private.

See Quantinuum/guppylang#1796

[maximilianruesch]

The flag for enabling experimental features can only be set globally, and not enabled as a context manager anymore: Quantinuum/guppylang#1515