bash: restore native variable completion fallback

[kudala-bharani]

Fixes #172.

Once shtab's completion function is registered, Bash stops completing variable names such as $BASH_V. Adding -o bashdefault lets Bash try its native completion when the generated function has no matches. This handles $VAR, ${VAR, and double-quoted forms without duplicating Bash's variable parsing. It also restores native tilde-user completion, but doesn't enable the separate filename fallback (-o default).

When file completion produces no candidates, the generated function also clears filenames mode before native fallback. Otherwise Bash 4+ escapes $ inside quoted variable names. Filename handling stays enabled when there are actual file candidates.

The tests press Tab in a real Bash session. The 24 variable cases cover positional arguments, --pid, file and directory options, --, and redirection. There's also a check that an ordinary filename prefix isn't completed as an option value. Removing the filename-mode reset reproduces six quoted-variable failures on Bash 5.2.

Validation:

• Linux, Python 3.14.7 / Bash 5.2: 119 passed, 67 skipped.
• macOS, Python 3.12.13 / Bash 3.2: 127 passed, 59 skipped.
• Coverage: 90.55% on both platforms (required: 85%).
• pre-commit run --all-files: passed.
• Existing directory trailing-slash, file completion, and redirection tests pass on Bash 5.2.

The generated Bash examples intentionally change to include the native fallback and filename-mode guard; other shell generators are unchanged.

[kudala-bharani]

Found the Linux failure: filename mode stayed enabled when no files matched, which made Bash escape $ during quoted variable completion. This now gets cleared before native fallback. I also added coverage for file and directory options. The full suite passes on Linux/Bash 5.2 and macOS/Bash 3.2, and pre-commit passes.