[Feature Request] Node.js 26 support (nodejs26.x)

**Summary**
Node.js 26 was [released](https://nodejs.org/en/blog/release/v26.0.0) on 2026-05-05 as the upcoming LTS version. This issue tracks the request to add official Node.js 26 support to the Lambda Runtime Interface Client.

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**Requested changes**
- Create a new `nodejs26.x` branch and release targeting the Node.js 26.x runtime
- Update NODE_MODULE_VERSION to 147
- Update CI matrix to build and test against Node.js 26

**Why this matters for this repo**
The RIC includes native (C++) bindings that are tightly coupled to the Node.js ABI version.
The V8 upgrade to v14.6 in Node.js 26 changes NODE_MODULE_VERSION to 147, meaning any native addon compiled against Node.js 24 will not load under Node.js 26 without a rebuild. Lambda customers using custom container images with the RIC need a nodejs26.x-compatible release to avoid runtime failures after upgrading.

**Notable Node.js 26 changes relevant to this repo**
- V8 updated to v14.6
- NODE_MODULE_VERSION bumped to 147
- TypeScript support is now stable by default (no longer requires --experimental-transform-types)
- Temporal API is now enabled by default

**Prerequisite:** https://github.com/aws/aws-lambda-base-images/issues/488 - Add Node.js 26 base image (public.ecr.aws/lambda/nodejs:26)

The Lambda base image for nodejs26.x must exist before this RIC release can be built and tested against the new runtime.