[w3c/DOM-Parsing] Prefix collision issues in serialization algorithms (Issue #75)

While revisiting the XML serialization spec I found a few more bugs in it that can lead to serialization issues:

# prefix redeclaration confusion

```
const root = document.appendChild(document.createElementNS('ns1', 'pre:root'));
const child = root.appendChild(document.createElementNS('ns2', 'pre:child'));
child.appendChild(document.createElementNS('ns1', 'grandChild'));
```

This currently results in `grandChild` being assigned the `pre` prefix without redeclaring it, which places it in `ns2` instead of `ns1`.

Firefox assigns a new `a0` prefix to `child` here to avoid this. Chrome redeclares the `pre` prefix on `grandChild` back to `ns2`.

Chrome's behavior seems to more closely match the author's intent in that it preserves all prefixes as specified. To match that, the algorithm should likely keep track of the prefix to namespace mapping in addition to the currently tracked inverse mapping, and use that to check if the prefix it wants to use needs to be redeclared.

# generated prefix collisions

```
const root = document.appendChild(document.createElementNS('ns1', 'ns1:root'));
root.setAttributeNS('ns2', 'attr', 'value');
```

This currently results in two declarations for the `ns1` prefix on the `root` element, one for `ns1` produced as part of serializing the element, one for `ns2` produced for the generated prefix `ns1`. That is not well-formed.

Firefox and Chrome both assigns a new prefix to `attr` here to fix this. In Firefox the prefix is `a0`, Chrome uses `ns2`.

To fix, the "generate a prefix" algorithm should probably check whether the prefix it intends to generate isn't declared already. If there is an existing local declaration for the prefix, it can loop to try a higher `prefix index`, matching Chrome's behavior.

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