LibWeb: Disambiguate GridTrackPlacement API

- Ambiguous `raw_value()` method is replaced with `line_number()` and
  `span()`.
- `line_name()` that before returned either line name or area name is
  replaced with `line_name()` and `area_name()`.
- `Position` type is replaced with `Line` and `Area` type so we don't
   have to guess while doing layout.

Affected test expectations:
- `template-lines-and-areas` - improvement over what we had before.
- `named-tracks` - rebaseline a giant test. will have to split it into
  smaller tests in the future.

Userland/Libraries/LibWeb/Layout/GridFormattingContext.h

@@ -1,4 +1,5 @@
 /*
+ * Copyright (c) 2023, Aliaksandr Kalenik <kalenik.aliaksandr@gmail.com>
  * Copyright (c) 2022-2023, Martin Falisse <mfalisse@outlook.com>
  *
  * SPDX-License-Identifier: BSD-2-Clause