enhancement: improve YAML syntax highlighting

The current YAML highlighting is missing a few common YAML constructs. Examples from the YAML 1.2.2 specification:

1. Special numbers: .inf, -.inf, .nan

   positive_inf: .inf
   negative_inf: -.inf
   not_a_number: .nan
   

   See YAML 1.2.2 §2.4, Example 2.20.

2. Hex, octal, and binary numbers

   hexadecimal: 0xC
   octal: 0o14
   binary: 0b1010
   

   See YAML 1.2.2 §2.4, Example 2.19.

3. Single-quoted strings and escaped quotes

   single: 'Hello, world!'
   escaped: 'It''s YAML'
   

   See YAML 1.2.2 §7.3.2.

4. Explicit tags

   string: !!str 123
   integer: !!int "42"
   

   See YAML 1.2.2 §6.9.1.

There are other YAML constructs as well, such as empty values, flow collections, anchors and aliases, and the document end marker. However, I don't think these necessarily need special highlighting support.

Start by locating the existing YAML syntax-highlighting entry point and its tests, then compare supported constructs with YAML 1.2.2 sections 2.4, 6.9.1, and 7.3.2. Add coverage for special numbers, hexadecimal/octal/binary numbers, single-quoted strings with escaped quotes, and explicit tags; done means these examples receive appropriate highlighting without regressing existing YAML cases.