fix: add newline when editing the .gitignore (#3229)

without a newline, appending to the `.gitignore` can result in problems

for example:

```
ntl init # => appends .netlify to .gitignore with no newline

echo ".cache" >> .gitignore
```

now the .gitignore looks like this:
```
.netlify.cache
```

adding a newline fixes this problem

Co-authored-by: Erez Rokah <erezrokah@users.noreply.github.com>
Co-authored-by: kodiakhq[bot] <49736102+kodiakhq[bot]@users.noreply.github.com>

src/utils/gitignore.js

@@ -12,7 +12,7 @@ const hasGitIgnore = async function (dir) {
 
 const ensureNetlifyIgnore = async function (dir) {
   const gitIgnorePath = path.join(dir, '.gitignore')
-  const ignoreContent = '# Local Netlify folder\n.netlify'
+  const ignoreContent = '# Local Netlify folder\n.netlify\n'
 
   /* No .gitignore file. Create one and ignore .netlify folder */
   if (!(await hasGitIgnore(dir))) {