Don't require isomorphic-fetch in environments with fetch

Requiring isomorphic-fetch swells the size of any packages / Lambdas using libsql by over 100%.

For example, here is the breakdown of a very simple Lambda using @libsql/client:

Looking at how these are related:

i.e. the primary dependencies introduced by isomorphic-fetch include:

I wonder what environments still need node-fetch anyway? Node 16 has been EOL'ed so Node can be assumed to have native fetch and I believe all versions of Deno and Bun have native fetch too.

Given the API already allows providing a fetch function, would it be possible to assume a native fetch and allow environments that don't have native fetch (or have a deficient/buggy version) to pass in isomorphic-fetch or somesuch as needed rather than have all users pay the cost of node-fetch and friends?

Start at the @libsql/client API's existing fetch option and its isomorphic-fetch dependency declaration. Trace how requests obtain fetch, then verify that environments with native fetch no longer pull in the dependency while callers can still provide an alternative fetch implementation when needed.