gh-37606: Border matrix
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Plain TeX users may remember `\bordermatrix`. Here we implement this as
options of the `Matrix` class's `str` method.
```
sage: M = matrix([[1,2,3], [4,5,6], [7,8,9]])
sage: M.subdivide(None, 2)
sage: print(M.str(unicode=True,
....: top_border=['ab', 'cde', 'f'],
....: bottom_border=['*', '', ''],
....: left_border=[1, 10, 100],
....: right_border=['', ' <', '']))
ab cde f
1⎛ 1 2│ 3⎞
10⎜ 4 5│ 6⎟ <
100⎝ 7 8│ 9⎠
*
```
Follow-up PR: As the guiding application for this feature, we equip
finite-dimensional modules with basis with methods `_repr_matrix`,
`_ascii_art_matrix`, `_unicode_art_matrix` that can be used as in this
example:
```
sage: M = matrix(ZZ, [[1, 0, 0], [0, 1, 0]],
....: column_keys=['a', 'b', 'c'],
....: row_keys=['v', 'w']); M
Generic morphism:
From: Free module generated by {'a', 'b', 'c'} over
Integer Ring
To: Free module generated by {'v', 'w'} over Integer
Ring
sage: M._unicode_art_ = M._unicode_art_matrix
sage: unicode_art(M) #
indirect doctest
a b c
v⎛1 0 0⎞
w⎝0 1 0⎠
```
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### :hourglass: Dependencies
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URL: https://github.com/sagemath/sage/pull/37606
Reported by: Matthias Köppe
Reviewer(s): David Coudert, Matthias Köppe
