How to highlight with a custom language and extension mapping

Hi,
I have a custom language (say foolang) registered in the hljs instance, is it possible to highlight a language that is not listed there:

diff2html/src/ui/js/highlight.js-helpers.ts

Line 164 in e5c8139

const languagesToExt: { [_: string]: string } = {

It is properly detected in the language variable (because the extension of the filename in the diff is .foolang):

diff2html/src/ui/js/diff2html-ui-base.ts

Lines 144 to 145 in e5c8139

    
           const language = file.getAttribute('data-lang'); 
        
           const hljsLanguage = language ? getLanguage(language) : 'plaintext';

But this code discards it for 'plaintext' since it is not listed in languagesToExt:

diff2html/src/ui/js/highlight.js-helpers.ts

Lines 653 to 655 in e5c8139

    
           export function getLanguage(fileExtension: string): string { 
        
             return languagesToExt[fileExtension] ?? 'plaintext'; 
        
           }

It would be convenient to be able to register custom language/extension mappings.

👋 Thanks for the contribution. Left a comment in the PR.

	const language = file.getAttribute('data-lang');
	const hljsLanguage = language ? getLanguage(language) : 'plaintext';

	export function getLanguage(fileExtension: string): string {
	return languagesToExt[fileExtension] ?? 'plaintext';
	}