ecs-logging-nodejs support multiple CommonJS, ESM, and TypeScript import styles

Support these import styles:

// 1. `const { ecsFormat } = require('@elastic/ecs-pino-format)` in JS and TS.
//    The preferred import style for JS code using CommonJS.
// 2. `import { ecsFormat } from '@elastic/ecs-pino-format'` in JS and TS.
//    ES module (ESM) import style. This is the preferred style for TypeScript
//    code and for JS developers using ESM.
// 3. `const ecsFormat = require('@elastic/ecs-pino-format')` in JS.
//    The old, deprecated import method. Still supported for backward compat.
// 4. `import ecsFormat from '@elastic/ecs-pino-format'` in JS and TS.
//    This works, but is deprecated. Prefer #2 style.
// 5. `import * as EcsPinoFormat from '@elastic/ecs-pino-format'` is TS.
//    One must then use `EcsPinoFormat.ecsFormat()`

This excludes support for the TypeScript-only "import = require" style:

import ecsFormat = require('@elastic/ecs-pino-format')

This change adds tests for each import style usage, which should help avoid the (recent, smallish) TypeScript PR rollercoaster.

This approach is based off what I see fastify doing: fastify docs on this and code.

Note that for TypeScript users I think this may be a breaking change. FWIW that TypeScript was only added in the last release (v1.3.0) and was the original intent was more for adding VSCode completion support than about TypeScript code integration.