Implement tab completion in REPL

[perlun]

Moved to GitLab

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Tab completion in the REPL with full IntelliSense/code completion would be an incredibly cool "wow!"-feature that would really make the REPL shine when compared to other tools (even though some of them have it already). Here's the status in other REPL environments for comparison:

• Ruby irb: supports tab-completion, but given the dynamic, untyped nature of the language, supporting a full, context- and type-aware code completion is hard. It does a fair job based on these preconditions though.
• Python (3.9) has some code completion but also seems to suffer from the shortcomings of the dynamic nature of the language.
• JShell (Java 11): This is where it starts getting interesting. JShell has very good code completion, by comparison. Try typing "foo".ind<TAB> and you'll see how it nicely helps you complete .indexOf(. Press one more time and you get nicely formatted Javadoc-based API descriptions of the method you are looking to call. :astonished: JShell is really an amazing piece of software that deserves more praise than it probably has received so far. :bow:
• csi (C# interactive) doesn't seem to support code completion in version 3.6.0-4.20224.5, which is a bit of a shame since it would make more useful.

[perlun]

See also #182.

(Side note, but it would be really nice with epics, "related issues" and other enterprise-oriented features in GitHub. Stuff that is already readily available in GitLab for the record. :stuck_out_tongue:)