Add support for pnpm

[uelibaumgartner]

Add pnpm as supported packager for client builds

This PR adds support for pnpm as a build packager in serverless-build-client.

Changes

• Added pnpm to the list of supported packagers.
• Added a default build command for pnpm (pnpm run build), consistent with the existing npm behavior.

Rationale

pnpm is widely used as a drop-in alternative to npm/yarn and is fully compatible with the current build execution model of this plugin.
The existing implementation already resolves packagers and commands dynamically, so no changes outside of the constants were required.

Usage

CLI:

serverless client build --packager pnpm

Via configuration:

custom:
    buildClient:
        packager: pnpm

Compatibility

• No breaking changes.
• Existing npm and yarn workflows are unaffected.
• Requires pnpm to be available in the runtime environment.