Usermanual: clusters chapter; add brief grapheme definition and clarify monotonous cluster handling.

Towards consolidating all array bsearch/...

Don't canonicalize '@' to '-' in language tags

Fixes #1406.

Geeza Pro for example, relies on that for fancy mark positioning.

Fixes https://github.com/harfbuzz/harfbuzz/issues/1405

[use] Fix reordering

Affects morx/kerx run-time only currently.  Will adjust their sanitize next.

Fixes https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=11400

See comment.

Supercedes https://github.com/harfbuzz/harfbuzz/pull/1401
Fixes https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=11366

This concludes null-size enforcement changes

See comments.

Since min_size is 0, Null() still accepts this type.

Cross-kerning can only take care of positioning vertically.  It doesn't
adjust mark advance...

Such that for Indic-like scripts (eg. Khmer), we don't do any fallback mark
advance-zeroing / positioning, but we do for Latin, etc.  Reuses preferences
of our script-specific OpenType shapers for those.

Fixes regression: https://github.com/harfbuzz/harfbuzz/issues/1393
Which means, fixes again: https://github.com/harfbuzz/harfbuzz/issues/1264
While not regressing: https://github.com/harfbuzz/harfbuzz/issues/1357