Add definition for 3.0.0-preview1

Rename 2.8.0 to 3.0.0

  https://github.com/ruby/ruby/commit/21c62fb670

Add definition for mruby 2.1.2

Add definition for JRuby 9.2.13.0

JRuby 9.2.13.0 has been released.
https://www.jruby.org/2020/08/03/jruby-9-2-13-0

Rename CONDUCT.md to CODE_OF_CONDUCT.md

Conforms to github's conventional filename

* '+' needs to be considered part of the name for --definitions and -l
  to see truffleruby and truffleruby+graalvm as separate.

* Supports using other GraalVM languages such as JavaScript, Python, R and Java.
* Supports running TruffleRuby on JVM and not only Native.
* Add GraalVM 20.1.0.

compatible with macOS 11

Add definition for JRuby 9.2.12.0

https://www.jruby.org/2020/07/01/jruby-9-2-12-0.html

Update rbx scripts and Added the latest definitions

Fix a bash 4.4 warning when installing jruby

add mruby 2.1.1

The existing install function wiped out the prefix that build_package_copy
copied the binaries to. The subsequently created symlinks point to files
that don't exist on disk.

The list `rbenv install -l` is too long, show latest stable releases by default

In particular, the following:

```
/path/to/bin/ruby-build: line 739: warning: command substitution: ignored null byte in input
```

Add artichoke-dev to install the latest Artichoke nightly build