fix($markdown): support path without file extension when importing code snippets (#2677)

Author: mitjat

packages/@vuepress/markdown/__tests__/__snapshots__/snippet.spec.js.snap

@@ -5,6 +5,10 @@ exports[`snippet import snippet 1`] = `
 // ..
 }
 </code></pre>
+<pre><code># Sample snippet file with no filename extension.
+FROM ubuntu:latest
+CMD echo hello!
+</code></pre>
 `;
 
 exports[`snippet import snippet with highlight multiple lines 1`] =`

packages/@vuepress/markdown/__tests__/fragments/Dockerfile

@@ -0,0 +1,3 @@
+# Sample snippet file with no filename extension.
+FROM ubuntu:latest
+CMD echo hello!

packages/@vuepress/markdown/__tests__/fragments/code-snippet.md

@@ -1 +1,3 @@
 <<< @/packages/@vuepress/markdown/__tests__/fragments/snippet.js
+
+<<< @/packages/@vuepress/markdown/__tests__/fragments/Dockerfile

packages/@vuepress/markdown/lib/snippet.js

@@ -137,7 +137,7 @@ module.exports = function snippet (md, options ={}){
  *
  * captures: ['/path/to/file.extension', 'extension', '#region', '{meta}']
  */
-constrawPathRegexp=/^(.+(?:\.([a-z]+)))(?:(#[\w-]+))?(?:?({\d+(?:[,-]\d+)*}))?$/
+constrawPathRegexp=/^(.+?(?:\.([a-z]+))?)(?:(#[\w-]+))?(?:?({\d+(?:[,-]\d+)*}))?$/
 
 constrawPath=state.src.slice(start,end).trim().replace(/^@/,root).trim()
 const[filename='',extension='',region='',meta='']=(rawPathRegexp.exec(rawPath)||[]).slice(1)