Add condense support to RequireJS plugin

This PR adds support for condensing to the RequireJS plugin.

See the test cases `test/condense/requirejs_*` for examples. Here's one example:

```json
{
  "!define": {
    "!requirejs": {
      "requirejs_const": {
        "!span": "7[0:7]-15[0:15]",
        "a": {
          "!span": "8[0:8]-9[0:9]",
          "!type": "string"
        }
      },
      "requirejs_dep": {
        "!span": "67[1:9]-87[1:29]",
        "a": "!requirejs.requirejs_const"
      }
    }
  },
  "!name": "requirejs_dep.js"
}
```