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/examples/memory/Program.cs

@@ -1,30 +1,30 @@
-using System;
-using Wasmtime;
-
-namespace HelloExample
-{
-    class Host : IHost
-    {
-        public Instance Instance { get; set; }
-
-        [Import("log")]
-        public void Log(int address, int length)
-        {
-            var message = Instance.Externs.Memories[0].ReadString(address, length);
-            Console.WriteLine($"Message from WebAssembly: {message}");
-        }
-    }
-
-    class Program
-    {
-        static void Main(string[] args)
-        {
-            using var engine = new Engine();
-            using var store = engine.CreateStore();
-            using var module = store.CreateModule("memory.wasm");
-            using dynamic instance = module.Instantiate(new Host());
-
-            instance.run();
-        }
-    }
-}
+using System;
+using Wasmtime;
+
+namespace HelloExample
+{
+    class Host : IHost
+    {
+        public Instance Instance { get; set; }
+
+        [Import("log")]
+        public void Log(int address, int length)
+        {
+            var message = Instance.Externs.Memories[0].ReadString(address, length);
+            Console.WriteLine($"Message from WebAssembly: {message}");
+        }
+    }
+
+    class Program
+    {
+        static void Main(string[] args)
+        {
+            using var engine = new Engine();
+            using var store = engine.CreateStore();
+            using var module = store.CreateModule("memory.wasm");
+            using dynamic instance = module.Instantiate(new Host());
+
+            instance.run();
+        }
+    }
+}