Officially document type checking behavior for operators WRT bigint

## Search Terms bigint operator type checking ## Suggestion I'd like to see official documentation of the design decision TS has made regarding type checking of operators now that bigint is in play. ## Use Cases The closure-compiler team is currently working on our support for BigInt. We want to make sure the type design choices we make there are compatible with TS going forward. It appears that right now when you perform an arithmetic operation on 2 `any` values, TS infers the result to be of type `number`, even in ESNext mode where one might expect the result to be `number|bigint`. https://www.typescriptlang.org/play/?target=99#code/DYUwLgBAhgXNB2BPCBeCBGeBuAUKSARnFEqhtnuBAMZwECWA5vfJGlBAFQQFZA I am inclined to do the same for closure-compiler, but I'd first like to make sure that this behavior was an intentional design choice for TS and not an oversight/bug that is likely to change in the near future. My point of view on this is that logically the `*` operator (for example) is overloaded, so there are really 2 versions of it. 1. The original `*` can take anything (except `bigint`), and always returns a `number`. 2. The new `bigint * bigint` version, which always returns a `bigint`, or throws an exception if one operand is not a `bigint`. The compiler should assume it's dealing with #1, unless type information indicates it is seeing #2. Insisting that all operands be clearly typed as `bigint` if you want `bigint` results prevents users from crossing the streams and accidentally mixing `bigint` and `number` values in computations. ## Examples 1. What is the result type of `bigint|number * bigint|number`? I expect `bigint|number` with no error message, since you may want to write methods that are intentionally meant to work for either case. 2. What is the result type of `bigint * any`? I expect the `bottom/can't happen` type, and an error message since we can't be sure that the right operand will be a `bigint` at runtime. 3. What is the result type of `any * any`? I…