Add support for Elvish transcripts.

[xiaq]

This adds support for highlighting transcripts of Elvish sessions.

Description

Support for Elvish was accepted in #5940. Because Elvish is a shell language, it is very frequent to not show just Elvish code, but also its outputs. Instead of just:

echo foo

One often needs to show:

~> echo foo
foo

These transcripts are most often used inside the Markdown of (for example) issue comments, and rarely saved to files, so there aren't a lot of those files on GitHub today.

However, my primary goal is to make it possible to use the elvish-transcript language tag on GitHub, and since elvish is already usable as a language tag, I'd argue that it should be supported too. The notability test was passed when Elvish itself was accepted.

I did define a file extension for Elvish transcripts, .elvts, but I am fine if I should drop that.

Checklist:

• I am adding a new language.
  • The extension of the new language is used in hundreds of repositories on github.com.
    • The .elvts extension is not widely used, see above for explanation.
  • I have included a real-world usage sample for all extensions added in this PR:
    • Sample source(s):
      • Examples given for the echo command (https://elv.sh/ref/builtin.html#echo, source)
    • Sample license(s): BSD 2-clause
  • I have included a syntax highlighting grammar: https://github.com/elves/elvish, already included
  • I have updated the heuristics to distinguish my language from others using the same extension.

[lildude]

We really don't need an extension or sample in this case and can simplify the entry (see inline comment).

Widespread usage is still a consideration which is quite easy to determine as you've stated this will only be used in markdown files or issue comments.

This search shows it is only being used in 135 markdown files so falls a little short on the files side.

Searching issue comments or bodies doesn't seem to indicate it's used that much with only 8 results.

So whilst there is usage, it doesn't appear widely adopted on GitHub yet, though I guess the lack of highlighting might have something to do with that in issues.

Let me sleep on this.

[xiaq]

Thanks, applied your suggested change. Also regenerated vendor/README.md and removed the sample.

I wonder though - how does Linguist know that the elvish-transcript language tag in Markdown maps to the source.elvish-transcript scope? Is it just based on the name of the scope? (Also FWIW, the reason I added an interpreter was based on the misunderstanding that it is also used as a language tag.)

So whilst there is usage, it doesn't appear widely adopted on GitHub yet, though I guess the lack of highlighting might have something to do with that in issues.

I definitely agree that the lack of usage currently is because there's no syntax highlighting yet, so people don't bother.

Looking at the first page of open issues in https://github.com/elves/elvish/issues, a lot of comments containing transcripts, but not using the elvish-transcript tag: elves/elvish#1599 (comment), elves/elvish#1603 (comment), elves/elvish#1625 (comment), elves/elvish#1631 (comment), elves/elvish#1641 (comment), elves/elvish#1648 (comment), elves/elvish#1653 (comment), elves/elvish#1654 (comment), elves/elvish#1656 (comment) (the last code block), elves/elvish#1662 (comment). That is about 1/3 of all Elvish issues that has at least one transcript and could benefit from elvish-transcript support. At least in the context of issues the use of Elvish transcripts is already widespread, just not the use of the elvish-transcript tag.

[lildude]

I wonder though - how does Linguist know that the elvish-transcript language tag in Markdown maps to the source.elvish-transcript scope? Is it just based on the name of the scope?

Linguist doesn't directly handle the codeblocks, markup does, but markup uses the languages and grammars supplied by Linguist.

I gave a brief run down of how this works just yesterday at #6293 (comment) but in short it knows because users tell it and thanks to the entry in the languages.yml file it knows to tell the syntax highlighting engine to use the grammar which has the source.elvish-transcript scope to highlight the blocks:

"The file/codeblock has been identified as 'Elvish Transcript', so Mr Syntax Highlighting Engine, please use the source.elvish-transcript grammar to make it pretty" 😁

[lildude]

Lets go for it.

[xiaq]

Awesome, thank you 🥳