fix: improve source map path normalization and monorepo support
- Add normalizeSourceUrl function to handle file:/ URL concatenation
- Fix bug where file:/ URLs at position 0 weren't handled (>= 0 instead of > 0)
- Add packages/next/dist to shouldIgnorePath for monorepo development
- Export shouldIgnorePath for reuse by webpack source map implementation
- Add unit tests for normalizeSourceUrl
