Add a .gitattributes file to specify eol=LF (#1370)

* Add a .gitattributes file specifying LF-style line endings.

This is similar to [Rust's .gitattributes file] though simplified.
Most of our source and documentation files already used LF-style line
endings, including *.cs files, so this makes things more consistent.

[Rust's .gitattributes file]: https://github.com/rust-lang/rust/blob/master/.gitattributes

* Remove UTF-8 BOMs in *.cs files.

Most of our *.cs files don't have UTF-8 BOMs, so this makes things more
consistent.

crates/misc/dotnet/tests/FunctionThunkingTests.cs

@@ -1,4 +1,4 @@
-using FluentAssertions;
+using FluentAssertions;
 using System;
 using System.Linq;
 using Xunit;