Typescript 5.5 disallows the "suppressImplicitAnyIndexErrors", so we
need to go through the code, find any places where an index has an
implicit "any" type, and annotate those places with the correct type so
that Typescript won't complain about them.

This commit makes a start on this task.

In a couple places, we do `if foo.hasOwnProperty(bar)` followed by
getting the value of `foo[bar]`, but Typescript warns that "string"
isn't a valid property name of `foo`. It should recognize that the
hasOwnProperty check means it's safe to access `foo[bar]`, but it
doesn't. We can either change those to be `if bar in foo` which
Typescript *does* recognize as a type guard, or else we can just leave
the code as-is and add @ts-expect-error comments. Since `bar in foo` is
not quite identical to `foo.hasOwnProperty(bar)` (`bar in foo` climbs
inheritance hierarchies whereas `.hasOwnProperty` doesn't), I chose to
leave the code as-is rather than subtly change its behavior.

Now that we've fixed the implicit "any" index errors, we can remove this
line from tsconfig. (It will be going away in Typescript 5.5 anyway).