Emit additional typings for ts4.2

[lukasIO]

uses downlevel-dts in the build step to emit type declarations compatible with ts4.2.
The reason for using 4.2 is that the ResizeObserver type isn't present in previous typescript versions, which results in a build error for versions <4.2.
Users that are stuck on typescript versions older than 4.2 can add "skipLibCheck": true to their tsconfig in order to make the library work (this is a recommended setting anyways and is part of the --init default tsconfig when creating a new project microsoft/TypeScript#37808).

[changeset-bot]

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Latest commit: c612baa

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