update expressions to error for non-ints

[tgauth]

PR Summary

• fix #390

PR Context

• existing grammar parses only the decimal digit(s) of floating point input as a number, resulting in incorrect handling by functions, like int()
• updates grammar to parse floating point input as a number, allowing Rust code to detect as invalid input when trying to parse as int (can be updated in the future, if floating point input should be supported)