feat: Add file filtering support (extension, glob, exclude)

[echobt]

This PR adds support for filtering search results by file extension, glob patterns (include/exclude), and file type categories.

Changes

• Added --ext to filter by file extension (e.g. --ext rs,toml)
• Added --glob to include files matching glob patterns (e.g. --glob "src/**/*.rs")
• Added --exclude to exclude files matching glob patterns (e.g. --exclude "target/**")
• Added --type to filter by predefined file categories (code, config, docs)
• Implemented FileFilter in src/core/filter.rs
• Updated SearchEngine and Database to apply filters
• Added unit tests for filtering logic

Example Usage

vgrep "query" --ext rs
vgrep "query" --glob "src/**/*.rs" --exclude "**/*test*"
vgrep "query" --type code