Use a mutable output buffer for Punycode encoding

[OskarEichler]

Summary

Initialize the Punycode encoder's output as an explicitly mutable string. Fixes #31 without changing encoding behavior or enabling frozen literals throughout the library.

Reproduction and verification

Ruby 4.0.6 with -W:deprecated reports that the output string literal will be frozen when encoding bücher. With --enable=frozen-string-literal, the same call raises FrozenError while appending the first basic code point.

After the change:

• The same -W:deprecated invocation produces the expected bcher-kva without a warning.
• Five encode/decode round trips pass with frozen literals enabled: ASCII, empty, mixed ASCII/Unicode, German and Japanese inputs.
• Existing rake test: 11 tests, 642 assertions, zero failures/errors on Ruby 4.0.6.
• Ruby syntax and git diff --check pass. The gem already requires Ruby >= 2.7, which supports unary String#+.

No repository tests were added or changed because this contribution's task explicitly prohibits test-file changes. Focused checks are external invocations; older Ruby versions and upstream CI remain unverified locally.

Compatibility / breaking changes

None intended. Encoded output, return type, dependencies and Ruby requirement are unchanged. Only the encoder's local output-buffer mutability is made explicit.

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