fix: handle empty newlines before diagrams in md

Handles empty newlines before and after diagrams in Markdown.

For example, the following Markdown:

````markdown
My text.

```mermaid
  my diagram
```
````

Should get converted to:

```markdown
My text.

![diagram](./my-diagram)
```

However, currently, the empty newline is removed.
We can fix this by making sure that our Regex ignores newlines outside
of the Mermaid diagram code.

Fixes: 9d7b4b2
Fixes: #413

src-test/test.js

@@ -316,6 +316,9 @@ describe("NodeJS API (import ... from '@mermaid-js/mermaid-cli')", () => {
         '(./mermaid-run-output-test-png-8.png "State diagram example with \\\\\\"double-quotes\\"")')
       expect(markdownFile).toContain(markdownImageWithCustomTitle)
 
+      // check whether newlines before/after mermaid diagram are kept
+      expect(markdownFile).toContain('There should be an empty newline after this line, but before the Mermaid diagram:\n\n')
+
       // files should exist, and they should be PNGs
       await Promise.all(expectedOutputPngs.map(async (expectedOutputPng) => {
         // markdown file should point to png relative to md file

src/index.js

@@ -364,7 +364,8 @@ async function run (input, output, { puppeteerConfig = {}, quiet = false, output
     }
   }
 
-  const mermaidChartsInMarkdown = /^\s*```(?:mermaid)(\r?\n([\s\S]*?))```\s*$/
+  // TODO: should we use a Markdown parser like remark instead of rolling our own parser?
+  const mermaidChartsInMarkdown = /^[^\S\n]*```(?:mermaid)(\r?\n([\s\S]*?))```[^\S\n]*$/
   const mermaidChartsInMarkdownRegexGlobal = new RegExp(mermaidChartsInMarkdown, 'gm')
   const browser = await puppeteer.launch(puppeteerConfig)
   try {

test-positive/mermaid.md

@@ -83,6 +83,9 @@ graph LR
     A -->|こんにちは| B
 ```
 7. sequence.mmd
+
+There should be an empty newline after this line, but before the Mermaid diagram:
+
 ```mermaid
 sequenceDiagram
 %% See https://mermaidjs.github.io/sequenceDiagram.html
@@ -103,6 +106,7 @@ loop D-1 before deadline at 7:45
     Note over HII,FGG: D-1 before deadline
 end
 ```
+
 8. Should still find mermaid code even when code-block is indented.
 
     ```mermaid