Add support for sed as a programming language

[Alhadis]

Description

This PR adds sed to GitHub's list of recognised programming languages. sed is Turing-complete, and has been used to program Tetris, calculators, and your average Turing machine.

In-the-wild usage

The number of sed scripts on GitHub is surprising: searching for .sed files brings back almost 600,000 results. All 959 samples I downloaded were unambiguously sed code.

Sample file

Locating an unencumbered sed script with a clear license was hard. Thankfully, FreeBSD's sed contained a decent-looking example of sed programming.

Licensed under the BSD-2-clause.

Language grammar

My own work, which I wrote especially for you guys. 😉

[Alhadis]

Oh yeah, I took the liberty of properly documenting my harvester script, which was moved from a gist to a repo after it needed an update. It's good enough to warrant mentioning in CONTRIBUTING.md, but I couldn't figure out which section was relevant enough to include it...

@lildude Did you know that in sed, it's possible to use #, ; and plain whitespace as delimiters for y and s? I didn't either until I wrote this grammar. 😁 Suffice to say getting these right wasn't easy.

I don't think I've written such an airtight grammar in my life...

$ echo 'upidly delimi' | sed -e 'stupidly delimited shit'
ed shi

[pchaigno]

This is getting boring @Alhadis. Could you at least mess up something in one of your pull requests from time to time?

EDIT: Never mind, you made a terrible mistake! There are actually 159k results in that search request instead of the 600k advertised. So low in-the-wild usage!

[lildude]

LGTM

[lildude]

@lildude Did you know that in sed, it's possible to use #, ; and plain whitespace as delimiters for y and s?

I did and have used them many a time to avoid having to escape the nuts out of strings, though I admit to never using a space - that sounds like pure sadism 😆.

[pchaigno]

Oh yeah, I took the liberty of properly documenting my harvester script, which was moved from a gist to a repo after it needed an update. It's good enough to warrant mentioning in CONTRIBUTING.md, but I couldn't figure out which section was relevant enough to include it...

👍 for adding it to the contribution guidelines!
I'd add it in the additional steps for both Adding an extension to a language and Adding a language, in place of "(ping @lildude to help with this)" maybe.