Add support for setting a .env file

The --env option is processed outside command js, because of the upcoming "cli configurator"  (configuration of the "core" application instance). Environment variables are simply injected into the process.env. This will then be returned by a custom bootstrapper for dealing with the env() calls, in custom configuration.

packages/cli/src/CliApplication.ts

@@ -5,12 +5,14 @@ import type {
 import type {
     Application
 } from "@aedart/contracts/core";
+import type { ParseArgsConfig } from "node:util";
 import { Application as CoreApplication } from "@aedart/core";
 import { CallbackWrapper } from "@aedart/support";
 import { Command as CommanderJs } from "commander";
 import SkipProcessExitError from "./exceptions/SkipProcessExitError";
 import { version } from "../package.json";
 import * as process from "node:process";
+import { parseArgs } from "node:util";
 
 /**
  * Cli Application
@@ -149,13 +151,15 @@ export default class CliApplication
      */
     public async run(argv?: readonly string[], options?: ParseOptions): Promise<boolean>
     {
+        this.processEnvOption(argv);
+
         return await this.core.run(CallbackWrapper.makeFor(
             this,
             this.parse,
             [argv, options]
         ));
     }
-
+    
     // TODO: 
     protected async parse(argv?: readonly string[], options?: ParseOptions): Promise<boolean>
     {
@@ -232,7 +236,11 @@ export default class CliApplication
     {
         return (new CommanderJs())
             .version(this.version)
-            .description(this.description);
+            .description(this.description)
+
+            // This option will be processed outside the "driver's" usual way of dealing with options!
+            // @see processEnvOption()
+            .option('--env <file>', 'set environment variables from supplied file.');
     }
 
     /**
@@ -267,4 +275,44 @@ export default class CliApplication
             throw err;
         })
     }
+
+    /**
+     * Processes the `--env` option for the cli application
+     * 
+     * @param {readonly string[]} [argv] Defaults to `process.argv` when no arguments given.
+     * 
+     * @protected
+     */
+    protected processEnvOption(argv?: readonly string[]): void
+    {
+        // Prepare options for the `parseArgs`.
+        const argsOptions: ParseArgsConfig = {
+            args: (argv as string[] | undefined),
+
+            // Define the '--env' option
+            options: {
+                'env': {
+                    type: 'string',
+                    //default: '.env'
+                }
+            },
+
+            // Ignore any other arguments and options...
+            strict: false,
+        }
+
+        // Parse the arguments and extract the values
+        const { values } = parseArgs(argsOptions);
+
+        // Resolve path to `.env` file.
+        const path = Reflect.has(values, 'env')
+            ? values['env']
+            : undefined;
+
+        // Load environment variables into `process.env`, if a path has been provided.
+        // If nothing was specified, then avoid doing anything.
+        if (typeof path === 'string' && path.length > 0) {
+            process.loadEnvFile(path);
+        }
+    }
 }