Allow createXlfFiles() respect existing <note> on xlf file.

[IlyaBiryukov]

We use createXlfFiles to generate ENU xlf file from 'package.nls.json', 'nls.metadata.header.json', and 'nls.metadata.json' files. Our loc team uses this ENU xlf file to produce translated xlfs.

The issue is that we have to have <note> elements in the trans units in our ENU xlf to pass instructions to the loc team, and createXlfFiles() just overwrites the ENU xlf, losing all the notes.

We had to work it around by getting the notes from the existing ENU xlf file first, and then applying them in our copy of createXlfFiles() as in this PR

I think this is a common scenario to be enabled in vscode-nls-dev