Only paste links when pasted text is only url

When the text being pasted contained a URL, we were attempting to convert the target text into a markdown link. However, this was causing some undeseried behavior, because the pasted text was being treated like an exact URL. This updates the RegExp test to look for an exact string of a URL so that we don't match on longer strings.

Borrowed from SO: https://stackoverflow.com/questions/3809401/what-is-a-good-regular-expression-to-match-a-url

Author: ipc103

Diff for: src/paste-markdown-link.ts

@@ -55,6 +55,7 @@ function linkify(selectedText: string, text: string): string {
   return `[${selectedText}](${text})`
 }
 
+const URL_REGEX = /https?:\/\/(www\.)?[-a-zA-Z0-9@:%._+~#=]{1,256}\.[a-zA-Z0-9()]{1,6}\/?\s*?$/i
 function isURL(url: string): boolean {
-  return /^https?:\/\//i.test(url)
+  return URL_REGEX.test(url)
 }

Diff for: test/test.js

@@ -43,6 +43,14 @@ describe('paste-markdown', function () {
       assert.equal(textarea.value, 'The examples can be found [here](https://github.com).')
     })
 
+    it('creates a markdown link when the pasted url includes a trailing slash', function () {
+      // eslint-disable-next-line i18n-text/no-en
+      textarea.value = 'The examples can be found here.'
+      textarea.setSelectionRange(26, 30)
+      paste(textarea, {'text/plain': 'https://www.github.com/'})
+      assert.equal(textarea.value, 'The examples can be found [here](https://www.github.com/).')
+    })
+
     it("doesn't paste a markdown URL when pasting over a selected URL", function () {
       // eslint-disable-next-line i18n-text/no-en
       textarea.value = 'The examples can be found here: https://docs.github.com'
@@ -66,6 +74,15 @@ describe('paste-markdown', function () {
       assert.equal(textarea.value, '@')
     })
 
+    it("doesn't paste markdown URL when additional text is being copied", function () {
+      textarea.value = 'github'
+      textarea.setSelectionRange(0, 6)
+      paste(textarea, {'text/plain': 'https://github.com plus some other content'})
+      // Synthetic paste events don't manipulate the DOM. The same textarea value
+      // means that the event handler didn't fire and normal paste happened.
+      assert.equal(textarea.value, 'github')
+    })
+
     it('turns html tables into markdown', function () {
       const data = {
         'text/html': tableHtml