Add concurrency limits

[giovannibenussi]

This PR to limits the number of parallel executions when calling the execute, batch, transaction, and executeMultiple methods. The number of parallel executions that can be executed is 20 by default, and can be customized by providing a new concurrency parameter when initializing the client as follows:

export const turso = createClient({
  url: process.env.TURSO_DATABASE_URL,
  authToken: process.env.TURSO_AUTH_TOKEN,
  concurrency: 10 // defaults to 20 and undefined means there's no concurrency limit
});

[giovannibenussi]

For context of why p-limit didn't work: it's a pure ESM package, which makes it really hard to work with Jest, even when following the official recommendations from Jest docs on the subject.

[giovannibenussi]

I added an update to the behavior of the concurrency value:

• undefined will be equivalent to 20 (same if the parameter is not provided).
• Negative numbers will be treated as 0, which means that there's no concurrency limit.
• Any positive number will limit the concurrency as expected.