Fix crash when assigning non-ASCII file extensions

[crleonard]

Assigning a non-ASCII extension, such as path.extension = "é", crashes with “Code point value does not fit into ASCII”.

Replace the ASCII-only scalar conversion in _makeExtension(_:) with the existing SystemString initializer, which uses the platform’s Unicode encoding. The assignment now produces the same path as constructing FilePath("file.é") directly.

Fixes #404

Tests and documentation

• Add regression coverage for accented characters, emoji, combining marks, and mixed ASCII/Unicode extensions.
• Cover hidden and compound filenames, extension replacement and removal, and unchanged special paths.
• Clarify Unicode support in the DocC comment with an example.

Validation

• Confirmed the original regression test crashes before the fix.
• Full macOS suite passes: 73 XCTest tests and 8 Swift Testing tests.
• Standalone Unicode reproducer passes against the optimized library.
• Linux and Windows runtime tests have not been run locally.