Add new line to the end of file, preserve line endings style

[pastelsky]

This PR fixes a couple of issues with bolt -

• Bolt is opinionated about the line endings style – it always uses \n to denote line endings. This PR enables bolt to automatically detect line endings style (\n vs \r\n) using detect-newline and preserve the style when updating / writing to a package.json file

• Whenever bolt writes a package.json, it now adds a newline character to the last line, in accordance with the POSIX definition of a line.

Note that these issues have already been fixed by npm (here) and yarn (here), and this just brings bolt's implementation on par.

Motivation

When using prettier or an opinionated editor alongside bolt, there is a constant push and pull between these tools that leads to ghost diffs like these –

[ajaymathur]

Hey @pastelsky , the change looks good. I have just added a comment for clarity :)

[pastelsky]

Updated the PR with os.EOL.