Replace naive string matching with proper glob library for exclude patterns

[dollfins]

Description

The shouldScan() method in diagnostics.ts implements a naive glob matcher that only handles **/ prefix patterns using string inclusion checks. This means complex exclude patterns like **/test_*, **/target/**, or **/fixtures/*.rs do not work correctly in all cases.

Current State

private shouldScan(filePath: string): boolean {
    const patterns = this.config.exclude.split(',');
    const relativePath = path.relative(workspaceFolder, filePath);
    return !patterns.some(pattern => {
        const glob = pattern.trim();
        if (glob.startsWith('**/')) {
            return relativePath.includes(glob.slice(3));
        }
        return false;
    });
}

Problems:

• relativePath.includes('test_*') treats * as a literal character, not a glob wildcard
• Patterns without **/ prefix are silently ignored (return false)
• Cannot exclude specific file extensions or directories properly

Expected Behavior

Use a proper glob matching library (or VS Code's built-in glob support) for exclude pattern evaluation:

1. Replace the naive implementation with minimatch (already compatible with VS Code patterns)
2. Support all standard glob patterns: *, **, ?, {a,b}, etc.
3. Validate patterns at configuration time and warn about invalid patterns
4. Consider using vscode.RelativePattern or vscode.GlobPattern for consistency with VS Code's own pattern matching

Implementation Notes

• minimatch is already a transitive dependency through VS Code types — add it as an explicit devDependency
• Or use picomatch for a lighter alternative
• Add unit tests for pattern matching with common exclude scenarios
• Cache compiled patterns for performance

Acceptance Criteria

• Pattern **/test_* excludes files like test_utils.rs and src/test_helper.rs
• Pattern **/target/** excludes all files in target directories
• Pattern **/fixtures/*.rs excludes Rust files in fixtures but not other file types
• Invalid patterns show a warning notification
• All existing tests pass
• No performance regression on workspace scan

Complexity

Trivial — replace a simple function with a well-tested library, add tests.

Points

100