Formatting support for trim trailing whitespace

[xorye]

The difference between files.trimFinalWhitespace and files.trimFinalNewlines, is that files.trimFinalNewlines will trim final newlines and spaces, while files.trimTrailingWhitespace removes trailing spaces on each line.

ie, if _ were spaces:

<a><a>\n
___\n
___\n
\n

would become

<a><a>\n
\n
\n
\n

Since vscode-xml supports files.trimFinalNewlines and files.insertFinalNewline, it would be cool to have files.trimTrailingWhitespace just so that it is consistent with the preferences VS Code has to offer.