Add QRCode.get_ascii() for string output

[NixMelon]

Summary

Closes #435.

This PR adds QRCode.get_ascii(), which returns the complete ASCII QR code representation as a string.

Previously, callers had to capture QRCode.print_ascii() output with io.StringIO or run the qr command through a subprocess. The new method allows terminal interfaces and other Python applications to use the generated text directly.

Changes

• Add QRCode.get_ascii(tty=False, invert=False) -> str.
• Preserve border, inversion, and TTY formatting behavior.
• Automatically compile the QR matrix when necessary.
• Refactor QRCode.print_ascii() to use get_ascii().
• Preserve print_ascii() stream validation, output, and flushing behavior.
• Replace the documented StringIO workaround with direct get_ascii() usage.
• Add the new API to the changelog.

Testing

The tests cover:

• returning a non-empty string;
• exact output parity with print_ascii();
• normal and inverted output;
• zero and standard borders;
• TTY escape codes;
• automatic QR compilation;
• repeated-call stability; and
• output dimensions.

Focused test result:

10 passed, 30 deselected