Check all intersection constituents against index signatures in target

[ahejlsberg]

With this PR we check that all constituents of an intersection source type relate to index signatures in the target type. For example:

declare let s: { a: string } & { b: number };
declare let t: { [key: string]: string };
t = s;  // Error

Previously, no error was reported above.

Fixes #32484.

[ahejlsberg]

@typescript-bot run dt
@typescript-bot test this

[typescript-bot]

Heya @ahejlsberg, I've started to run the extended test suite on this PR at 63ed5b0. You can monitor the build here. It should now contribute to this PR's status checks.

[typescript-bot]

Heya @ahejlsberg, I've started to run the parallelized Definitely Typed test suite on this PR at 63ed5b0. You can monitor the build here. It should now contribute to this PR's status checks.

[ahejlsberg]

Closing in favor of #35143.