shader_file: add .glsl to the list of recognized extensions

In the default resource pack, recent Minecraft versions (namely, 1.17)
have additional shader files at assets/minecraft/shaders/include with
the .glsl extension, which contain GLSL code that can be imported in
other fragment and vertex shaders via the #moj_import preprocessor
directive.

PackSquash didn't recognize .glsl as a valid shader extension, so it
skipped those files and broke resource packs which modified .glsl files
and included shaders which relied on those modified files.

Let's treat .glsl as a valid shader extension, alongside .fsh and .vsh.

src/pack_file/shader_file.rs

@@ -148,7 +148,7 @@ impl<T: AsyncRead + Unpin + 'static> PackFileConstructor<T> for ShaderFile<T> {
 	) -> Option<Self> {
 		let extension = to_ascii_lowercase_extension(args.path.as_ref());
 
-		if matches!(&*extension, "fsh" | "vsh") {
+		if matches!(&*extension, "fsh" | "vsh" | "glsl") {
 			file_read_producer().map(|(read, file_length_hint)| Self {
 				read,
 				// The file is too big to fit in memory if this conversion fails anyway