Part of issue #343.

This commit causes an alias to an invalid command to be highlighted as an error
(unknown-token).

This allows a caller to know if the command or process substitution is
complete.

No functional change.

No functional change.

Prepares for next commit. No functional change.

Fixes #494

This will allow for correct path_prefix highlighting in backticks.

This allows for callees to prepend highlights before $reply after the
length of the feature (e.g. command substution) is known.

This will allow for highlighting $( ) and similar.

No functional change.

No functional change.

No functional change.

Fixes #539.

main: Probe dirs after blacklist

Move tests on the highlight path until after the blacklist is checked so
that blacklisted paths are not tested. This should prevent hangs on dead
remote filesystems provided the path is blacklisted in
X_ZSH_HIGHLIGHT_DIRS_BLACKLIST.

Updated tests to 'default' highlight as the blacklist is now checked
before the path is tested.

Bug: https://github.com/zsh-users/zsh-syntax-highlighting/issues/528

zsh/macOS bug: http://www.zsh.org/cgi-bin/mla/redirect?WORKERNUMBER=42891

docs: Fix typos

The «=(<foo)» construct was highlighted in blue (globbing?) from the '>' to the ')'.