Improve Deno compatibility: config-first and safe env access

[Jobians]

This PR introduces three changes to make the pg package more compatible with Deno while keeping full Node.js functionality:

1. Default user value

   • Replace user: process.platform === 'win32' ? process.env.USERNAME : process.env.USER
     with user: 'postgres' in defaults.
   • Avoids requiring environment variables in Deno.

2. Config-first parameter resolution

   • Update val() in connection-parameters.js to return config[key] first, before checking environment variables.
   • Prevents Deno errors when --allow-env is not granted.

3. Safe NODE_PG_FORCE_NATIVE check

   • Wrap the NODE_PG_FORCE_NATIVE check in a try/catch.
   • Ensures process.env access in Deno doesn’t throw, while preserving Node.js behavior.

These changes maintain Node.js compatibility, preserve the lazy-loading of the native module, and allow using the package in Deno without requiring --allow-env.