Enhance `view-markdown-source` with line actions #2542

I also understand that links like "https://github.com/sindresorhus/refined-github/blob/master/contributing.md#L76" only work with RG installed. Non RG users will just be brought to the top of the file. (That is a shortcoming of GH with markdown.)

This feature appears to be super easy. This is the code needed, it just needs to be cleaned up (use .append, JSX, our icons, drop unnecessary attributes)

1. Visit: https://github.com/sindresorhus/refined-github/blob/3d517d6cd00e83a51973808c7ff03cae2b692d2b/contributing.md#L

2. Run this code

   $('.blob-wrapper').insertAdjacentHTML('beforeend', `<details class="details-reset details-overlay BlobToolbar position-absolute js-file-line-actions dropdown d-none" aria-hidden="true" >
    <summary class="btn-octicon ml-0 px-2 p-0 bg-white border border-gray-dark rounded-1" aria-label="Inline file action toolbar" aria-haspopup="menu" role="button">
      <svg class="octicon octicon-kebab-horizontal" viewBox="0 0 16 16" version="1.1" width="16" height="16" aria-hidden="true"><path d="M8 9a1.5 1.5 0 100-3 1.5 1.5 0 000 3zM1.5 9a1.5 1.5 0 100-3 1.5 1.5 0 000 3zm13 0a1.5 1.5 0 100-3 1.5 1.5 0 000 3z"></path></svg>
    </summary>
    <details-menu role="menu">
      <ul class="BlobToolbar-dropdown dropdown-menu dropdown-menu-se mt-2" style="width:185px">
        <li>
          <clipboard-copy role="menuitem" class="dropdown-item" id="js-copy-lines" style="cursor:pointer;" tabindex="0" value="import select from 'select-dom';" data-ga-click="Blob, copyLines, numLines:1">Copy line</clipboard-copy>
        </li>
        <li>
          <clipboard-copy role="menuitem" class="dropdown-item" id="js-copy-permalink" style="cursor:pointer;" tabindex="0" value="https://github.com/sindresorhus/refined-github/blob/3d517d6cd00e83a51973808c7ff03cae2b692d2b/source/features/view-markdown-source.tsx#L3" data-ga-click="Blob, copyPermalink, numLines:1">
            Copy permalink
          </clipboard-copy>
        </li>
        <li><a class="dropdown-item js-update-url-with-hash" id="js-view-git-blame" role="menuitem" href="https://github.com/sindresorhus/refined-github/blame/3d517d6cd00e83a51973808c7ff03cae2b692d2b/source/features/view-markdown-source.tsx#L3" data-ga-click="Blob, viewGitBlame, numLines:1">View git blame</a></li>
          <li><a class="dropdown-item" id="js-new-issue" role="menuitem" href="https://github.com/sindresorhus/refined-github/issues/new?permalink=https%3A%2F%2Fgithub.com%2Fsindresorhus%2Frefined-github%2Fblob%2F3d517d6cd00e83a51973808c7ff03cae2b692d2b%2Fsource%2Ffeatures%2Fview-markdown-source.tsx%23L3" data-ga-click="Blob, newIssue, numLines:1">Reference in new issue</a></li>
      </ul>
    </details-menu>
   </details>`)

3. Click on a line (or multiple lines) like you normally would