docs: update README and lib.rs for namespaced variables

[AprilNEA]

Summary

This PR adds support for namespaced variables with dot notation (e.g., ${project.DATABASE_URL}). This feature allows users to organize variables using prefixes or hierarchical naming schemes.

Changes

• Added comprehensive test coverage for namespaced variables:

  • namespaced_vars: Tests basic namespaced variable substitution
  • multiple_namespaced_vars: Tests multiple namespaced variables in a single template
  • deeply_namespaced_vars: Tests deeply nested namespaced variables

• Updated documentation:

  • Added description of namespaced variable support in README.md
  • Added example usage in README.md
  • Added documentation and example in lib.rs module documentation

Implementation Details

The existing implementation already supported dots in variable names since the validation function only prohibited $, {, and } characters. This PR formally documents and tests this capability.