Add isTransferring check to transfer hook examples

[Woody4618]

This check checks the source token account if it is currently transferring to make sure that the transfer hook can only be called during a transfer.
This is to make sure that for example a counter or similar can not be manipulated by calling the transfer hook directly.

[ZYJLiu]

update the cargo.toml dependency from

anchor-lang = { git = "https://github.com/coral-xyz/anchor", branch = "master",features = ["interface-instructions"] }
anchor-spl = { git = "https://github.com/coral-xyz/anchor", branch = "master" }

back to

anchor-lang = { version = "0.30.0", features = ["interface-instructions"] }
anchor-spl = "0.30.0"

looks good and learned that TransferHookAccount state existed!

[Woody4618]

Thank you :)