unicode/bidi: fix Direction zero value to Neutral

[pgundlach]

The Direction constants had LeftToRight as iota (value 0), making it
indistinguishable from the zero value of an unset Direction field.
This caused DefaultDirection(LeftToRight) to be silently ignored in
Paragraph.Order(), as the code could not distinguish between "user
explicitly set LTR" and "no direction was set."

Reorder the Direction constants so that Neutral is the zero value,
and add an explicit case for LeftToRight in Order() to set the
paragraph embedding level to 0.

Fixes golang/go#71809

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[gopherbot]

Message from Gopher Robot:

Patch Set 1:

(1 comment)

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[gopherbot]

Message from Patrick Gundlach:

Patch Set 2:

(2 comments)

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[gopherbot]

Message from Patrick Gundlach:

Patch Set 2:

(1 comment)

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