Make experimental-features a proper type

Rather than having them plain strings scattered through the whole
codebase, create an enum containing all the known experimental features.

This means that
- Nix can now `warn` when an unkwown experimental feature is passed
  (making it much nicer to spot typos and spot deprecated features)
- It’s now easy to remove a feature altogether (once the feature isn’t
  experimental anymore or is dropped) by just removing the field for the
  enum and letting the compiler point us to all the now invalid usages
  of it.

src/libexpr/primops/fetchTree.cc

@@ -176,7 +176,7 @@ static void fetchTree(
 
 static void prim_fetchTree(EvalState & state, const Pos & pos, Value * * args, Value & v)
 {
-    settings.requireExperimentalFeature("flakes");
+    settings.requireExperimentalFeature(Xp::Flakes);
     fetchTree(state, pos, args, v, std::nullopt, FetchTreeParams { .allowNameArgument = false });
 }
 
@@ -290,7 +290,7 @@ static RegisterPrimOp primop_fetchTarball({
       The fetched tarball is cached for a certain amount of time (1 hour
       by default) in `~/.cache/nix/tarballs/`. You can change the cache
       timeout either on the command line with `--option tarball-ttl number
-      of seconds` or in the Nix configuration file with this option: ` 
+      of seconds` or in the Nix configuration file with this option: `
       number of seconds to cache `.
 
       Note that when obtaining the hash with ` nix-prefetch-url ` the
@@ -393,7 +393,7 @@ static RegisterPrimOp primop_fetchGit({
           ```
 
           > **Note**
-          > 
+          >
           > It is nice to always specify the branch which a revision
           > belongs to. Without the branch being specified, the fetcher
           > might fail if the default branch changes. Additionally, it can
@@ -430,12 +430,12 @@ static RegisterPrimOp primop_fetchGit({
           ```
 
           > **Note**
-          > 
+          >
           > Nix will refetch the branch in accordance with
           > the option `tarball-ttl`.
 
           > **Note**
-          > 
+          >
           > This behavior is disabled in *Pure evaluation mode*.
     )",
     .fun = prim_fetchGit,