Make ecmascript.ts type-safe:

* Improves types for slots and intrinsics
* ecmascript.ts now has types throughout and compiles with
  `noImplicitAny` (although a later PR will actually enable this
  option)
* Fixes BigInt errors in a few places to fix compiler errors and
  test failures. Will be replaced by James's work on JSBI.
* Fixes a few index.d.ts bugs in the public types.
* Overhauls how plural units are handled in TS types.
* Partially adds types to calendar.ts. (More coming later.)
* A new internal types file for non-public shared types.

Author: justingrant

.eslintrc.yml

@@ -94,7 +94,5 @@ overrides:
   - files:
       - test/*.mjs
       - test/**/*
-      # TODO Fix these files!
-      - lib/ecmascript.ts
     rules:
       no-param-reassign: off