Replace getPrimaryKey w/ getId

[KyleAMathews]

Working on the design for #79 made me realize getPrimaryKey is a lousy function name.

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Latest commit: c64fccc

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[pkg-pr-new]

examples/react/todo

npm i https://pkg.pr.new/@tanstack/db@80

npm i https://pkg.pr.new/@tanstack/db-collections@80

npm i https://pkg.pr.new/@tanstack/react-db@80

commit: c64fccc