modularize codebase

[pkra]

As per F2F with @zorkow, first order of business is to modularize the current code base.

[pkra]

Here's a first step towards this: we split chromify.js into three modules:

• helpers.mjs exports makeId
• rewriteNode.mjs: exports rewriteNode (imports makId, also has its specific helpers)
• navigator.mjs: exports attachNavigator (imports makId, also has its specific helpers)
• main.mjs which is just

import {rewriteNode} from '/rewriteNode.mjs';
import {attachNavigator}  from '/navigator.mjs';

document.querySelectorAll('[data-semantic-structure]').forEach((node, index)=>{
    rewriteNode(node, index);
    attachNavigator(node, index);
})

And adapt index.html to load main.mjs as a module.

[pkra]

Afterwards we can continue thinking about eliminating makeId and fixing aria-owns calculation.