Improve JS logging (#743)

This PR tries to improve and rationalize what we log. Key points:
- introduce `logger.ts`: each file/component is encouraged to use the logger instead of writing directly to `console.*`
    * the logger automatically prepend a prefix like `[py-config]`, `[py-env]` which make it easier to understand where a certain message is printed from
    * it provide a central place where to add more features in the future. E.g., I can imagine having a config setting to completely silence the logs (not implemented yet)
- use the new loggers everywhere
- write to `.info()` instead of `.log()`. The idea is to keep `console.log` free, so that for the users it's easier to tell apart their own messages and the pyscript ones
- generally improve what we log. This is an endless exercise, but I tried to print more things which are useful to understand what's going on and in which order the various things are executed, and remove prints which were clearly debugging leftovers

pyscriptjs/src/logger.ts

@@ -0,0 +1,64 @@
+/* Very simple logger interface.
+
+   Each module is expected to create its own logger by doing e.g.:
+
+       const logger = getLogger('my-prefix');
+
+   and then use it instead of console:
+
+       logger.info('hello', 'world');
+       logger.warn('...');
+       logger.error('...');
+
+   The logger automatically adds the prefix "[my-prefix]" to all logs; so e.g., the
+   above call would print:
+
+       [my-prefix] hello world
+
+   logger.log is intentionally omitted. The idea is that PyScript should not
+   write anything to console.log, to leave it free for the user.
+
+   Currently, the logger does not to anything more than that. In the future,
+   we might want to add additional features such as the ability to
+   enable/disable logs on a global or per-module basis.
+*/
+
+interface Logger {
+    debug(message: string, ...args: any[]): void;
+    info(message: string, ...args: any[]): void;
+    warn(message: string, ...args: any[]): void;
+    error(message: string, ...args: any[]): void;
+}
+
+const _cache = new Map<string, Logger>();
+
+function getLogger(prefix: string): Logger {
+    let logger = _cache.get(prefix);
+    if (logger === undefined) {
+        logger = _makeLogger(prefix);
+        _cache.set(prefix, logger);
+    }
+    return logger;
+}
+
+function _makeLogger(prefix: string): Logger {
+    prefix = "[" + prefix + "] ";
+
+    function make(level: string) {
+        const out_fn = console[level].bind(console);
+        function fn(fmt: string, ...args: any[]) {
+            out_fn(prefix + fmt, ...args);
+        }
+        return fn
+    }
+
+    // 'log' is intentionally omitted
+    const debug = make('debug');
+    const info = make('info');
+    const warn = make('warn');
+    const error = make('error');
+
+    return {debug, info, warn, error};
+}
+
+export { getLogger };