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/global/Program.cs

@@ -1,32 +1,32 @@
-using System;
-using Wasmtime;
-
-namespace HelloExample
-{
-    class Host : IHost
-    {
-        public Instance Instance { get; set; }
-
-        [Import("print_global")]
-        public void PrintGlobal()
-        {
-            Console.WriteLine($"The value of the global is: {Global.Value}.");
-        }
-
-        [Import("global")]
-        public readonly MutableGlobal<int> Global = new MutableGlobal<int>(1);
-    }
-
-    class Program
-    {
-        static void Main(string[] args)
-        {
-            using var engine = new Engine();
-            using var store = engine.CreateStore();
-            using var module = store.CreateModule("global.wasm");
-            using dynamic instance = module.Instantiate(new Host());
-
-            instance.run(20);
-        }
-    }
-}
+using System;
+using Wasmtime;
+
+namespace HelloExample
+{
+    class Host : IHost
+    {
+        public Instance Instance { get; set; }
+
+        [Import("print_global")]
+        public void PrintGlobal()
+        {
+            Console.WriteLine($"The value of the global is: {Global.Value}.");
+        }
+
+        [Import("global")]
+        public readonly MutableGlobal<int> Global = new MutableGlobal<int>(1);
+    }
+
+    class Program
+    {
+        static void Main(string[] args)
+        {
+            using var engine = new Engine();
+            using var store = engine.CreateStore();
+            using var module = store.CreateModule("global.wasm");
+            using dynamic instance = module.Instantiate(new Host());
+
+            instance.run(20);
+        }
+    }
+}