Add first draft of default attribute definitions

[eemeli]

Adds an initial set of expression, markup, and message attribute definitions.

The proposed attributes are drawn from:

• XLIFF 2.2
• The messages.json web extension definition for placeholders.example
• https://github.com/eemeli/message-resource-wg/issues/19

As noted in the text, this is not intended as a final list, but as a starting point. The text is not being currently proposed to be normative, but we could change that later.