Add configurable backtrace_additional_directories option

[mitchh456]

Summary

• Add backtrace_additional_directories config option to include custom directories in backtrace parsing
• Backtraces now match paths in configured directories in addition to the default lib/, app/, and config/
• Useful for Rails engines, components, or other non-standard directory structures

Configuration

YAML (scout_apm.yml):

production:
  backtrace_additional_directories:
    - engines
    - components

Environment variable:
SCOUT_BACKTRACE_ADDITIONAL_DIRECTORIES='["engines", "components"]'

Test plan

• Existing backtrace parser tests pass
• New tests for empty additional directories (default behavior)
• New tests for single and multiple additional directories
• New tests verify default directories still work when additional dirs configured
• New tests verify special regex characters in directory names are properly escaped