Clarify type validation error message for == expression

@domlet is already working on this.

Since Jul 9, 2021.

@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.