[C++29] Introduce a higher level of organization to [lex]

[AlisdairM]

This issue is a specific suggestion to improve the presentation of [lex] for C++29 as part of #7399.

Introduce two new level-2 subclasses to better group the current level-2 subclauses to reflect their place in phase 3 of translation 3, and phase 7 of translation. Note that phase 4 of translation is the whole of [cpp].

• 5 Lexical convensions
  • 5.1 Separate translation
  • 5.2 Phases of translation
  • 5.3 Characters
    • 5.3.1 Character sets
    • 5.3.2 Universal character names
  • 5.4 Comments
  • 5.5 Preprocessor tokenization
    • 5.5.1 Preprocessing tokens
    • 5.5.2 Header names
    • 5.5.3 Preprocessing numbers
    • 5.5.4 Operators and punctuators
    • 5.5.5 Alternative tokens
  • 5.6 Translation unit tokenization
    • 5.6.1 Tokens
    • 5.6.2 Identifiers
    • 5.6.3 Keywords
    • 5.6.4 Literals
      • 5.6.4.1 Kinds of literals
      • 5.6.4.2 ...

The subdivisions under the new subclauses largely match the re-ordering of subclauses for C++26, #7316.