Add autocompletion for Keywords in textutils.complete()

[JakobDev]

At the moment, textutils.complete will not complete Keywords. But this PR add a 3rd Argument with allows to enable autocompletion with Keywords.

[Wojbie]

Could you provide use case for this addition?

[JakobDev]

Lua shell or edit

[SquidDev]

@Wilma456 This doesn't appear to do what you want it to do. There are a couple of problems:

• This reintroduces the do completing to dofile issue (which was previously fixed by 76a3562).
• This won't complete keywords on the "global" level (so i will not complete to if).
• If you have a field with a keyword (such as x = { ["if"] = 1 }) then x. will complete to x.if. However, this is a syntax error and so is probably not intentional behaviour.