Add some clarity to section 1 "How TypeScript Works"

[Kevin Diem]

Given this section discusses transpilation I think it would help to remove a reference to jsx as it can't be run directly in the browser and, very similar to TypeScript itself, needs transpiled down to JS.

With a JavaScript-only project, you would typically write your code in files with a .js or .jsx file extension. These files are then able to be directly executed in the browser or a runtime environment like Node.js (which is used to run JavaScript on servers, or on your laptop). The JavaScript you write is the JavaScript that gets executed.