Auto merge of #52644 - varkor:lib-feature-gate-2, r=withoutboats

Add errors for unknown, stable and duplicate feature attributes

- Adds an error for unknown (lang and lib) features.
- Extends the lint for unnecessary feature attributes for stable features to libs features (this already exists for lang features).
- Adds an error for duplicate (lang and lib) features.

```rust
#![feature(fake_feature)] //~ ERROR unknown feature `fake_feature`

#![feature(i128_type)] //~ WARNING the feature `i128_type` has been stable since 1.26.0

#![feature(non_exhaustive)]
#![feature(non_exhaustive)] //~ ERROR duplicate `non_exhaustive` feature attribute
```

Fixes #52053, fixes #53032 and address some of the problems noted in #44232 (though not unused features).

There are a few outstanding problems, that I haven't narrowed down yet:
- [x] Stability attributes on macros do not seem to be taken into account.
- [x] Stability attributes behind `cfg` attributes are not taken into account.
- [x] There are failing incremental tests.

src/librustdoc/lib.rs

@@ -13,17 +13,14 @@
        html_root_url = "https://doc.rust-lang.org/nightly/",
        html_playground_url = "https://play.rust-lang.org/")]
 
-#![feature(ascii_ctype)]
 #![feature(rustc_private)]
 #![feature(box_patterns)]
 #![feature(box_syntax)]
-#![feature(fs_read_write)]
 #![feature(iterator_find_map)]
 #![feature(set_stdio)]
 #![feature(slice_sort_by_cached_key)]
 #![feature(test)]
 #![feature(vec_remove_item)]
-#![feature(entry_and_modify)]
 #![feature(ptr_offset_from)]
 #![feature(crate_visibility_modifier)]
 #![feature(const_fn)]