Work on reflection groups

[stumpc5]

The work on reflection groups is in:

• #11187: main ticket with the first implementation
• #20395: (merged into #11187) preparation for the category framework to demand the method degrees.
• #20396: (merged into #11187) Implementation of the invariant form for crg's.
• #20477: Reduced words in crg's.
• #20445: Iteration algorithm using coset decomposition
• #20484: cythonizing has_descent, first_descent, descents. Code is available in #20445
• #20521: left/right actions for real and primal/dual for all reflection groups
• #25467: discriminant up- and downstairs
• #25466: Ambient space transformation
• #25477: iteration through general permutation groups (used for non-real reflection groups)

Other improvements:

• #20440: Field of definition
• #20444: Fake degree polynomial
• #20448: Generators of module of derivations
• #25478: Fix bug in is_regular
• #25483: Implement Coxeter numbers in the sense of Malle
• #25479: Jacobian and primitive vector field
• #25485: Braid orbit of reduced words in Coxeter group

Related tickets:

• #20402: Get subword complexes to work with reflection groups
• #20943: Updated the c flip implementation that was forgotten in #20402
• #20420: Dual braid monoid/group and Hecke algebras
• #18631: Rational Cherednik algebras (for finite Weyl groups)
• #20469: Ariki-Koike algebras
• #17303: Specht modules for Hecke algebras
• #25487: bug in minimal working example of Coxeter groups

Gap3, Chevie, and the raw data:

• #20107: gap3 experimental package
• #20393: gap3 optional package
• #20401: Hard-coded stuff

[vdelecroix]

update milestone 8.3 -> 8.4