Add Python and JavaScript rules to .editorconfig

[ferdnyc]

This builds on #3029, and adds some basic Python and JavaScript style rules:

Python

• UTF-8 encoding
• Four-space indents
• No trailing whitespace

JavaScript

• UTF-8 encoding
• Two-space indents
• No trailing whitespace

These settings seem to be roughly in line with the existing codebase. (The indentation on the JavaScript code is actually a complete mess, it switches seemingly at random between four-, two-, and one-space indents mixed with tab indents.)

[jonoomph]

LG