Add support for chain simulation

[bartolomej]

Description

Add support for emulating testnet and mainnet networks, using --chain-id emulator flag.

See: https://github.com/onflow/flow-emulator

TODO

• Fix account index lookup (use the correct chain ID)
• Fix default account indexing (generate default accounts using index lookup and the correct chain ID)
• Optionally support --start-block-height flag
• Add UI to switch or set chain (ideally not buried under workspace settings)