Adding best practices for defining identifiers [I18N-ACTION-1128]

[aphillips]

• added charmod-norm requirements not previously imported
• added explanations and examples links to each item
• modified identifier_case to provide equivalent guidance

NOTE WELL: identifier_non_ascii_namespace is entirely new and is based on our comments on CSS Variables.

Next steps:

Add some glue text.

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[aphillips]

@r12a have a look at the most recent changes (for my action from this week). These are more speculative and I'd value a second opinion.