add a interface for process launching

This is to allow external usage of the library to control process launching (and, for one example, place processes we launch into special cgroups.)

src/DefaultProcess.ts

@@ -0,0 +1,44 @@
+// Default process implementation.
+// This uses execa like the current code, but conforms to our abstration :)
+
+import EventEmitter from "events";
+import { IProcess, IProcessLauncher, ProcessLaunchOptions } from "./ProcessInterface";
+import { execaCommand } from "execa";
+import { Readable, Writable } from "stream";
+
+
+class DefaultProcess extends EventEmitter implements IProcess {
+    private process;
+    stdin: Writable | null = null;
+    stdout: Readable | null = null;
+    stderr: Readable | null = null;
+
+    constructor(command: string, opts?: ProcessLaunchOptions) {
+        super();
+
+        this.process = execaCommand(command, opts);
+
+        this.stdin = this.process.stdin;
+        this.stdout = this.process.stdout;
+        this.stderr = this.process.stderr;
+
+        let self = this;
+        this.process.on('spawn', () => {
+            self.emit('spawn');
+        });
+
+        this.process.on('exit', (code) => {
+            self.emit('exit', code);
+        });
+    }
+
+    kill(signal?: number | NodeJS.Signals): boolean {
+        return this.process.kill(signal);
+    }
+}
+
+export class DefaultProcessLauncher implements IProcessLauncher {
+    launch(command: string, opts?: ProcessLaunchOptions | undefined): IProcess {
+        return new DefaultProcess(command, opts);
+    }
+}

src/ProcessInterface.ts

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+import {type Stream, EventEmitter, Readable, Writable} from 'node:stream';
+
+export type StdioOption =
+	| 'pipe'
+	| 'overlapped'
+	| 'ipc'
+	| 'ignore'
+	| 'inherit'
+	| Stream
+	| number
+	| undefined;
+
+// subset of options. FIXME: Add more!!!
+export interface ProcessLaunchOptions {
+    stdin?: StdioOption,
+    stdout?: StdioOption,
+    stderr?: StdioOption
+}
+
+export interface IProcess extends EventEmitter {
+    stdin: Writable | null;
+    stdout: Readable | null;
+    stderr: Readable | null;
+
+
+    // Escape hatch; only use this if you have no choice
+    //native() : any;
+
+    kill(signal?: number | NodeJS.Signals): boolean;
+
+}
+
+// Launches a processs.
+export interface IProcessLauncher {
+    launch(command: string, opts?: ProcessLaunchOptions) : IProcess;
+}