Scientific notation literals should not support decimal exponents

[mathiaspeters]

Describe the bug
Specifying a literal like 1e2.5 should produce and error but currently just ignores what's after the decimal point.

Expected behavior
An error should be produced and the result field should reflect that

Additional context
The way this is currently implemented is that nom only looks for integers for the exponent so the parse doesn't even see the decimal. A better solution would be to look also for floating point numbers and have a specific validation step before converting to f64. Or just include the decimal exponent when parsing to f64 and let rust take care of it.