C API: Rework and enhance the `PyUnicodeWriter` implementation

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Placeholder issue to keep track of changes to rework and enhance the PyUnicodeWriter implementation.

Linked PRs

• gh-158451: Add _PyUnicodeWriter_SetBuffer() helper function #158452
• gh-158451: Add _PyUnicodeWriter_WriteLatin1String() test #158464
• gh-158451: Convert _PyUnicodeWriter_Prepare() to static inline function #158466
• gh-158451: Reuse PyUnicodeWriter_WriteStr() in PyUnicode_Format() #158467
• gh-158451: Fix python-gdb.py for invalid Unicode strings #158475
• gh-158451: Add _PyUnicodeWriter_FinishWithSize() test #158481
• [3.14] gh-158451: Fix python-gdb.py for invalid Unicode strings (GH-158475) #158495
• [3.15] gh-158451: Fix python-gdb.py for invalid Unicode strings (GH-158475) #158496
• gh-158451: Optimize PyUnicode_Join() #158693