WriteLinesToFile Does Not Support BOM Customization

## Context
The WriteLinesToFile task uses UTF-8 encoding without BOM (byte order mark) by default. This is not specified in [the documentation](https://docs.microsoft.com/visualstudio/msbuild/writelinestofile-task) and could be useful to have control over.

The difficulty is allowing control over the optional `Encoding` parameter _and_ control over the BOM without adding a special case for every single encoding.

Some options I sent in the internal thread that started this issue:
1. We pick a few encodings that are most common/popular/etc. and special case those and no others.
1. We can pass an optional parameter to override whether s_defaultEncoding applies a BOM.

Let's discuss what potential solutions there are before implementing an improvement.