sagemathgh-40420: Method A-field for Drinfeld modules
    
We implement the method `A_field` for Drinfeld modules.
It is a synonym of `base`; however, having it will be convenient when we
implement Anderson motives (coming soon) since A-field and base ring are
not the same in this case.


### 📝 Checklist

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- [x] The title is concise and informative.
- [x] The description explains in detail what this PR is about.
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- [x] I have created tests covering the changes.
- [x] I have updated the documentation and checked the documentation
preview.
    
URL: sagemath#40420
Reported by: Xavier Caruso
Reviewer(s): Antoine Leudière

sagemathgh-40430: Use the variable name τ instead of t for Drinfeld modules
    
In the theory of Drinfeld module, the classical name of the
noncommutative variable is `τ`. However, in Sage, we currently use `t`,
which might conflict with other standard notations (often the variable
name of the underlying function ring is denoted by `t`).
In this PR, we propose to shift to the notation `τ`. It just changes how
the outputs are printed (by default), not the interface.

### 📝 Checklist

<!-- Put an `x` in all the boxes that apply. -->

- [x] The title is concise and informative.
- [x] The description explains in detail what this PR is about.
- [ ] I have linked a relevant issue or discussion.
- [x] I have created tests covering the changes.
- [x] I have updated the documentation and checked the documentation
preview.

### ⌛ Dependencies

sagemath#40420, sagemath#40421
    
URL: sagemath#40430
Reported by: Xavier Caruso
Reviewer(s): Antoine Leudière