[TM-293] limit annotations' allowed characters

Description

The set of allowed character for annotations is different from the one allowed for strings. Parsing and Annotation constructors are incorrect and should instead only accept annotations that match this regexp (from Michelson's docs):

@%|@%%|%@|[@:%][_a-zA-Z][_0-9a-zA-Z\.%@]*

Related issue(s)

https://issues.serokell.io/issue/TM-293

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