Add BufferLogAppender

Sometimes it is useful to capture log messages in a way that is easy for
the running program to introspect or manually handle.
*BufferLogAppender* captures the logs in a simple array.

Be careful, there is no default flush/clear mechanism that runs
automatically. Users of BufferLogAppender should take care to
render/handle log messages and periodically call `clearBuffer` on the
*BufferLogAppender* instance to avoid the buffer array growing without
limit.

src/buffer-log-appender.ts

@@ -0,0 +1,26 @@
+import type {
+  LogLevel,
+  LogMessage
+} from './log-message'
+
+export class BufferLogAppender implements LogAppender {
+  public threshold = LogLevel.ALL
+  public buffer: LogMessage[]
+
+  constructor(buffer: LogMessage[], threshold?: LogLevel) {
+    this.buffer = buffer
+
+    if (threshold) {
+      this.threshold = threshold
+    }
+  }
+
+  public appendMessage(msg: LogMessage): void {
+    if (this.threshold && msg.level < this.threshold) return
+    else buffer.push(msg)
+  }
+
+  public clearBuffer(): void {
+    this.buffer.length = 0
+  }
+}