fix regex in src/client/fileindex.js for FireFox

https://lively-kernel.org/lively4/lively4-jens/src/client/fileindex.js

  if (file.url.match(/\.md$/)) {
      // #BUG prevents loading in #FireFox due to invalid regexp group
      // FF RegExp version does not support lookbehinds. https://stackoverflow.com/questions/49816707/firefox-invalid-regex-group
      // #TODO Refactor

      // let patternMdFiles = /(?<=(\]:\s*)|(\]\s*\())((http(s)?:\/\/(w{3}[.])?([a-z0-9.-]{1,63}(([:]{1}[0-9]{4,})|([.]{1}){1,}([a-z]{2,})){1,})([a-zA-Z0-9\/\.\-\_#.?=%;]*))|((([./]+|[a-zA-Z\-_]))([a-zA-Z0-9\-_]+\.|[a-zA-Z0-9\-_]+\/)+([a-zA-Z0-9\-_#.?=%;]+)?))/gm
      //      // /(?<=<|\[.*\]:\s*|\[.*\]\)|src\s*=\s*('|")|href\s*=\s*('|"))((((http(s)?:\/\/)(w{3}[.])?)([a-z0-9-]{1,63}(([:]{1}[0-9]{4,})|([.]{1}){1,}([a-z]{2,})){1,}))|([./]+|[a-zA-Z_-]))([a-zA-Z0-9\-_]+\.|[a-zA-Z0-9\-_]+\/)+((\.)?[a-zA-Z0-9\-_#.?=%;]+(\/)?)/gm
      // extractedLinks = file.content.match(patternMdFiles)
      
      
    } else if (file.url.match(/\.(css|(x)?html)$/)) {
      
      // #TODO Refactor
      
      // let patternHtmlCssFiles = /(?<=(src\s*=\s*|href\s*=\s*|[a-zA-Z0-9\-_]+\s*\{\s*.*\s*:\s*)('|"))((((http(s)?:\/\/)(w{3}[.])?)([a-z0-9-]{1,63}(([:]{1}[0-9]{4,})|([.]{1}){1,}([a-z]{2,})){1,}))|([./]+|[a-zA-Z\-_]))([a-zA-Z0-9\-_]+\.|[a-zA-Z0-9\-_]+\/)+((\.)?[a-zA-Z0-9\-_#.?=%;]+(\/)?)/gm
      // extractedLinks = file.content.match(patternHtmlCssFiles)
    }

Start in src/client/fileindex.js and inspect the commented Markdown and HTML/CSS link-matching patterns, focusing on the lookbehind groups that Firefox rejects. Reproduce the file-loading path in Firefox and verify that link extraction still works for the supported file types without an invalid regular expression.