Commit b4e0de8

In order to make the resulting tool available to the widest possible audience, it is web-native. HTML, CSS, and JavaScript means it should work in most browsers and platforms. Wherever possible, if something about the implementation began to be too complex, I took that as a sign that I wasn't doing it "the web way" and tried to re-think how to leverage the tools built into the browser better. Modern web browsers are powerful platforms, with a rich set of tools for building great apps, worth exploring for projects large and small.

An important distinction between web applications and traditional desktop or server applications is the lack of a `main()` or other entry point. There is not explicit run loop because that is already built into the browser and implicit on every web page. All our code will be parsed and executed on load, at which point we can register for events we are interested in for interacting with the user. After the first run, all further interaction with our code will be through callbacks we set up and register, whether we register those for events (like mouse movement), timeouts (fired with the periodicity we specify) or frame handlers (called for each screen redraw, generally 60 frames per second). The browser does not expose full-featured threads (only shared-nothing Web Workers) either.

Each block consists of a few HTML elements, styled with CSS, with some JavaScript event handlers for drag-and-drop and modifying the input argument. This file helps to create and manage these grouping of elements as single objects. When a type of block is added to the block menu, it is also associated with a JavaScript function to run to implement the language, and so each block in the script has to be able to find its associated function and to call it when the script runs.