This PR finishes the Typescript migration of the Firestore Server SDK. As part of this:

• I changed the Validator class to use free-standing functions. This gets rid of the dynamic method creation, which doesn't really work with strong typing
• I changed the client to use unknown instead of any for the user-facing types (only internally, not in the types we shipped)
• I brought the client more in line with the Web SDK and only support the direction/query operator spelling that is outlined in the type definition (for example, "ASC" no longer works). This lets me use a common property validation function (and is the only breaking change)
• I removed the dependency on is to be able to use TypeScript's typechecking.

This is the PR that I am planning on merging, and it's also the PR that I am would like to address all feedback in (to reduce merge conflicts). Since it is not very easy to review, I split this out into a series of PRs that are more tightly scoped:

• Cleanup: Argument count validation #495 Cleanup: Argument count validation
• Cleanup: Validate primitive types #496 Cleanup: Validate primitive types
• Cleanup: Move parseGetAllArguments #497 Cleanup: Move parseGetAllArguments
• Cleanup: Enforce Import ordering #498 Cleanup: Enforce Import ordering
• Cleanup: Remove validator #499 Cleanup: Remove validator
• Cleanup: Remove 'any' #500 Cleanup: Remove 'any'
• Cleanup: Remove is #503 Cleanup: Remove is
• refactor: Add strict types for the transaction test #504 Cleanup: Add strict types for the transaction test
• Cleanup: Field value validation #505 Cleanup: Field value validation
• Cleanup: Misc #506 Cleanup: Misc
• Cleanup: Error messages for custom objects #507 Cleanup: Error messages for custom objects
• Cleanup: Enum validation #508 Cleanup: Enum validation

The individual PRs are not meant to compile/pass tests. There should not be anything in this larger PR (which should pass CI, fingers crossed) that is not part of the other PRs.

This is fairly low priority, so please take your time to review.

Fixes: b/117464450 and b/119350730