Disable ^C async workaround for zsh version 5.8 and later

Update INSTALL.md

[fix] install this plugin aside other plugins

Fixes/flaky ci

To reproduce:

1. Run `zsh -f`.
2. Run this:
  function bye() { BUFFER=bye }
  zle -N bye
  bindkey '^B' bye
  print -s 'hibye unexpected'
  source ~/zsh-autosuggestions/zsh-autosuggestions.zsh
3. Type `hi` and press Ctrl-B.

Expected: POSTBUFFER is empty.

Actual: POSTBUFFER is " unexpected".

There's something funny occasionally happening when `with_history` is
used twice in the same test. It seems to be happening more frequently
since asynchronous mode was enabled by default. My guess is it has
something to do with the `C-c` keys being sent toward the end not
consistently terminating the prompt. But I'm really not sure how it
would ever get into a `then` block like it seems to:

```
Failure/Error: wait_for { session.content }.to eq('echo "hello\nworld"')

  expected: "echo \"hello\\nworld\""
       got: "then> echo \"hello\\"
```

Sticking to only one `with_history` per terminal session (per test)
seems to fix the flakiness.

I also removed an old test case because I could not understand why it
was necessary and so couldn't write a good description for it. Could be
we'll need to add it back in at some point.

Write mock history to a temp file and load it directly with `fc -R`
instead of running each command individually to build up the history.

Enable async mode by default in newer versions of zsh

cleanup: Use more idiomatic method of checking if var is set

Be more specific about the built-in widgets we want to avoid wrapping

Add links to documentation on zsh glob patterns to the readme

See GitHub issue #503

Allow users to override the default by unsetting (or setting) the
ZSH_AUTOSUGGEST_USE_ASYNC variable.

See GitHub issue #498.

To avoid wrapping the built-in widgets (e.g. `autosuggest-fetch`,
`autosuggest-toggle`), we were ignoring all widgets whose names start
with `autosuggest-`. This had the downside of preventing wrapping of
user-defined widgets whose names happened to also start with that
prefix.

By being more specific about the exact built-in widgets we want to avoid
wrapping, we can allow users to define widgets whose names start with
`autosuggest-`.

See GitHub issue #496.

We are already using this method in other places.

For example: `ZSH_AUTOSUGGEST_USE_ASYNC`

v0.6.4

Allow skipping completion suggestions when buffer matches a pattern

Call original widget before moving cursor when accepting suggestion

cleanup: Split on null bytes instead of pattern matching

The check on length of `$POSTDISPLAY` is in support of the test for
`vi-delete` on the last char of the buffer with `dl`.

Fixes issue #482.

Set ZSH_AUTOSUGGEST_COMPLETION_IGNORE to a glob pattern to have the
completion suggestion strategy never make suggestions when the buffer
matches the pattern.

This can be helpful when some completion routines you have are
particularly expensive and you want to prevent them from running
automatically on every keystroke.

See GitHub issue #463.

(README) Link to 256 color chart

Easy to see and choose a color, and then copy it over from the chart.

Follow-up to technique tried in commit d94475ca after I learned about the
`@` flag to keep empty strings in the array.

We are getting errors on circle ci builds (e.g. see [1]):

```
CircleCI was unable to run the job runner because we were unable to
execute commands in build container.

This typically means that the build container entrypoint or command is
terminating the container prematurely, or interfering with executed
commands.  Consider clearing entrypoint/command values and try again.
```

Folks in this thread [2] suggest removing the `command: /sbin/init` line
initially added by the v1 => v2 migration script.

---

1: https://circleci.com/gh/zsh-users/zsh-autosuggestions/381
2: https://discuss.circleci.com/t/circleci-was-unable-to-run-the-job-runner/31894/18