Rollup merge of #48235 - varkor:parse-float-lonely-exponent, r=alexcrichton

Rollup merge of #48235 - varkor:parse-float-lonely-exponent, r=alexcrichton

Make ".e0" not parse as 0.0

This forces floats to have either a digit before the separating point, or after. Thus `".e0"` is invalid like `"."`, when using `parse()`. Fixes #40654. As mentioned in the issue, this is technically a breaking change... but clearly incorrect behaviour at present.