Add support for subcommand extensions

[szkiba]

This PR implements native support for k6 subcommand extensions in xk6, enabling developers to build and run subcommand extensions more efficiently.

Changes

This PR adds two key capabilities to xk6:

1. Subcommand Execution (xk6 x)

Introduces the xk6 x command that builds a k6 binary with the extension from the current directory and immediately executes the custom subcommand.

Usage:

# Build and run the subcommand extension
xk6 x <subcommand_name> [flags]

# Example: run the my-tool subcommand with debug flag
xk6 x my-tool --debug

This mirrors the k6 v1.5.0+ syntax (k6 x subcommand) and eliminates the need to manually build and execute binaries during development.

2. Subcommand Extension Scaffolding (xk6 new --type subcommand)

Extends the xk6 new command to generate boilerplate code for subcommand extensions.

Usage:

xk6 new --type subcommand my-extension

The generated template follows the official grafana/xk6-subcommand-example structure, providing a ready-to-use starting point for building custom k6 subcommands.

Motivation

With k6 v1.5.0 introducing subcommand extension support, developers can now create custom CLI commands under the k6 x namespace. However, the development workflow required manual binary builds for each test cycle:

xk6 build --with .
./k6 x my-subcommand

This PR streamlines the workflow to match existing xk6 patterns (like xk6 run for JavaScript extensions), making subcommand extension development more efficient.

Related

Closes #416