babel-code-frame: Highlight strings with green (not red) (babel#4579)

- Red makes it look like something is wrong with the string.
- On Ubuntu-based systems, it looks kinda broken.
- The error markers (`>` and `^`) as well as invalid tokens are already
  marked with red. By not having strings red, the most important
  information -- the error location -- is more visible.

This is a continuation of commit fa1de5c (PR babel#4572).

packages/babel-code-frame/src/index.js

@@ -13,7 +13,7 @@ let defs = {
   punctuator:  chalk.yellow,
   // bracket:  intentionally omitted.
   number:      chalk.magenta,
-  string:      chalk.red,
+  string:      chalk.green,
   regex:       chalk.magenta,
   comment:     chalk.grey,
   invalid:     chalk.white.bgRed.bold,