Documentation: fix linter errors with TypeScript declarations in Vitest setup instructions

[MaddyGuthridge]

ESLint (with the typescript-eslint plugin) gives a no-undef error without the <T = any> constraint in the type definitions. This PR adds the constraint to help prevent people from getting linting errors from the configuration. TypeScript doesn't seem to be fussed either way, but this should save people a minor annoyance.

What

Add type variable to TypeScript + Vitest setup instructions in the documentation.

Why

Prevent minor annoyances when linting fails for people who use the definitions.

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Latest commit: 9792571

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