webassembly: Add JavaScript proxying, and js and jsffi modules.

This commit improves the webassembly port by adding:

- Proxying of Python objects to JavaScript with a PyProxy type that lives
  on the JavaScript side.  PyProxy implements JavaScript Proxy traps such
  as has, get, set and ownKeys, to make Python objects have functionality
  on the JavaScript side.

- Proxying of JavaScript objects to Python with a JsProxy type that lives
  on the Python side.  JsProxy passes through calls, attributes,
  subscription and iteration from Python to JavaScript.

- A top-level API on the JavaScript side to construct a MicroPython
  interpreter instance via `loadMicroPython()`.  That function returns an
  object that can be used to execute Python code, access the Python globals
  dict, access the Emscripten filesystem, and other things.  This API is
  based on the API provided by Pyodide (https://pyodide.org/).  As part of
  this, the top-level file is changed from `micropython.js` to
  `micropython.mjs`.

- A Python `js` module which can be used to access all JavaScript-side
  symbols, for example the DOM when run within a browser.

- A Python `jsffi` module with various helper functions like
  `create_proxy()` and `to_js()`.

- A dedenting lexer which automatically dedents Python source code if every
  non-empty line in that source starts with a common whitespace prefix.
  This is very helpful when Python source code is indented within a string
  within HTML or JavaScript for formatting reasons.

Signed-off-by: Damien George <damien@micropython.org>

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