Prevents some flakiness in tests

Based on e702ec4697/async.zsh (L400-L406)

Checks whether `zpty` gives a file descriptor, which was not the case in
older versions of zsh.

Based on a4b2f81c96/async.zsh (L395-L401)

As far as I know, `fc` makes it impossible to tell whether history items
used an actual newline character or the string "\n". Pulling from the
`$history` array gives a more accurate representation of the actual
command that was run.

v0.3.3

Disable autosuggest if buffer is too large

Make buffer max size configurable, defaulted to infinity

According to a few tests, the `fc` builtin appears to be quite a bit
faster than searching through the `$history` associative array when
dealing with large history files (500K+).

Document caveats of match_prev_cmd strategy

This strategy relies on the history being exactly in the order in which
commands have been entered.  Therefore, options like suppressing
duplicates or expiring duplicates first will lead to unexpected
suggestions.