Main plan for clause reorganization for C++29

[tkoeppe]

This issue is a living record of the developing plan for reorganizing clauses for C++29.

• Move [cpp] closer to [lex] and rework [lex]. See [DO NOT MERGE] Proposed Core reorganization for C++29 #7272 and Prepare C++26 for clause reorganization in C++29 #7352 for suggestions. We'd like a paper for the more profound changes in [lex].
  • [lex.separate][module.unit] move definitions of program and translation unit #7180
• Dissolved [except] and split it across various other core clauses. See [C++29][except] Dissolve the exceptions clause into the rest of the document #7317, Prepare C++26 for clause reorganization in C++29 #7352 for suggestions.

The above points need to be worked out in detail.

Further, unrelated suggestions for reorganization welcome.