[dom] consider renaming "namespace" params into "namespaceURI" (#136)

Change signature of methods like
```
Attr? getNamedItemNS(DOMString? namespace, DOMString localName);
```
into
```
Attr? getNamedItemNS(DOMString? namespaceURI, DOMString localName);
```

Not a really important change, but `namespace` is a keyword in some programming languages;
plus `namespaceURI` is the name used in the API itself (`ElementnamespaceURI`, `NodelookupNamespaceURI`, etc)

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Reply to this email directly or view it on GitHub:
https://github.com/whatwg/dom/issues/136