Autocomplete language for code block that targets Code playground with "custom language name" not supported by Pygments

[i-ky]

Zulip autocompletes language of the code block:

zulip/static/js/composebox_typeahead.js

Lines 612 to 633 in 9c18641

	// Start syntax highlighting autocompleter if the first three characters are ```
	const syntax_token = current_token.slice(0, 3);
	if (this.options.completions.syntax && (syntax_token === "```" || syntax_token === "~~~")) {
	// Only autocomplete if user starts typing a language after ```
	if (current_token.length === 3) {
	return false;
	}
	// If the only input is a space, don't autocomplete
	current_token = current_token.slice(3);
	if (current_token === " ") {
	return false;
	}
	// Trim the first whitespace if it is there
	if (current_token[0] === " ") {
	current_token = current_token.slice(1);
	}
	this.completing = "syntax";
	this.token = current_token;
	return Object.keys(pygments_data.langs);
	}

...but suggestions are limited to languages supported by Pygments (pygments_data.langs).

Zulip also allows users to configure Code playgrounds with "custom language names":

You can also use a custom language name to implement simple integrations. For example, a code block tagged with the "language" send_tweet could be used with a "playground" that sends the content of the code block as a Tweet.

I think these "custom language names" should also be suggested by autocomplete feature.

For example, if send_tweet playground is configured in the organization and users types

```se

...then Zulip should suggest send_tweet.

[alya]

Thanks for the suggestion @i-ky -- makes sense to me!

[tridentpancake]

@zulipbot claim

[zulipbot]

Hello @tridentpancake, it looks like you've currently claimed 1 issue in this repository. We encourage new contributors to focus their efforts on at most 1 issue at a time, so please complete your work on your other claimed issues before trying to claim this issue again.

We look forward to your valuable contributions!