Livecode Builder is a new dialect of LiveCode in which you are able to write things called extensions. An extension will (in the first instance) either be a library or a widget - livecode.com 
Extensions come in two flavours:
Libraries are collections of commands and functions that integrate into the engine in exactly the same way as engine commands and functions do (the only caveat being initially is that you will be restricted to generic function / command call syntax to access them. Full and proper syntax bindings will have to wait until Open Language is born).
Widgets are collections of commands, functions and event handlers that together create a new control type, or widget.
The intention is not to create two LiveCode languages, and longer term you will be able to translate Livecode scripts automatically as needed (with a script translation tool) we are planning as part of the syntax cleanup project that will be enabled by Open Language).
The main aim of this clean up is to ensure we have a solid, predictable, reliable and completely defined base to work from without having to worry about any hangovers from the (very much organically evolved) past.
Livecode Builder extensions will be based upon a "new dialect" of LiveCode (let's call it LiveCodeish.
Emscripten is a source-to-source compiler (Transpiler), that transforms LLVM bytecode, typically created by compiling from C or C++, to JavaScript. It produces a subset of JavaScript known as asm.js - wikipedia