Support pasting URLs over markdown text (#29566)

Support pasting URLs over selection text in the textarea editor. Does
not work in EasyMDE and I don't intend to support it. Image paste works
as usual in both Textarea and EasyMDE.

The new `replaceTextareaSelection` function changes textarea content via
[`insertText`](https://developer.mozilla.org/en-US/docs/Web/API/Document/execCommand#using_inserttext)
command, which preserves history, e.g. `CTRL-Z` works and is also
demostrated below. We should later refactor the image paste code to use
the same function because it currently destroys history.

Overriding the formatting via `Shift` key is supported as well, e.g.
`Ctrl+Shift+V` will insert the URL as-is, like on GitHub.


![urlpaste](522b1023-6797-401c-9e4a-498570adfc88)

web_src/js/utils/url.js

@@ -1,3 +1,15 @@
 export function pathEscapeSegments(s) {
   return s.split('/').map(encodeURIComponent).join('/');
 }
+
+function stripSlash(url) {
+  return url.endsWith('/') ? url.slice(0, -1) : url;
+}
+
+export function isUrl(url) {
+  try {
+    return stripSlash((new URL(url).href)).trim() === stripSlash(url).trim();
+  } catch {
+    return false;
+  }
+}