Add additional Burmese styles

[r12a]

See w3c/sealreq#50

Additions include

• preferred alternatives for (prefix and) suffix.
• 2 alphabetic styles

[r12a]

https://www.w3.org/TR/predefined-counter-styles/#myanmar-styles now has default prefix&suffix assignments for the myanmar and shan styles, and a separate note to indicate that authors may want to replace those by different assignments. The defaults were changed to parentheses, rather than a period suffix.

This avoids the need to create new style templates in this document for each and every combination of base algorithm and styles of suffix/prefix (which could generate a large number of alternatives).

This should resolve the proposed addition "preferred alternatives for (prefix and) suffix".

What remains for this issue is to clarify and verify the algorithms for the alphabetic styles described in w3c/sealreq#50.