Add lexer for IEC 61131-3 Structured Text

[tali]

The new lexer uses sets of keywords to quickly categorize names and uses regular expressions for other elements like numbers and punctuation.

Closes #2024

[tali]

This is still in draft, I'd still like to work on disambiguation and include better example code.
I already created this PR to be able to obtain some feedback about the current state.
Especially feedback from smalltalk experts would be helpful for disambiguation of *.st files.
Ruby is not my native language, so please also have a closer look that I did everything right.

[tali]

(just rebased to current master)

[bufferoverflow]

@tali I really like this addition! I guess this is no longer draft and ready for maintainer review?

[tali]

This code passes all my tests and is able to highlight our production code. From my side it is ready.
@jneen, @gfx, what do you think? Can we merge it?

[tali]

Hi @tancnle, can you have a look please? I'd like to get this into the next release so that we can use it in our company GitLab.

[bufferoverflow]

@dblessing could you maybe review this? Would be a great addition to GitLab!

[tancnle]

@tali @bufferoverflow I will have a look at it sometime today.

[tancnle]

Thanks, @tali. LGTM 🚀