Clarify type validation error message for == expression

[anandthakker]

@ChrisLoer points out that the documentation for "==" could be a bit confusing. The docs say:

Equality is strictly typed: values of different types are always considered not equal

That is true for, e.g., ["==", ["get", "x"], 10] applied to {properties: {x: "10"}}. However, it's not true for an expression like ["==", "10", 10], which actually fails to typecheck with the message Cannot compare string and number. This error message appears to be in contradiction with the docs.

Let's clarify the message to something like:

Cannot compare ${toString(lhs.type)} and ${toString(rhs.type)}: since these types are different, "==" would always return false. Consider using "to-${toString(lhs.type)}" or "to-${toString(rhs.type)}" in order to compare two values of the same type.

[anandthakker]

Another option here is to simply collapse ["==", "10", 10] to false (instead of failing validation), maybe emitting a warning in case it's due to typo