Export base-bash-libs version constant

[codeforester]

Summary

• Add readonly BASE_BASH_LIBS_VERSION when lib_std.sh is sourced.
• Read the value from the repository/package VERSION file so that file remains the source of truth.
• Document the version constant in the root README and stdlib README.

Validation

• RED: bats --filter "stdlib exposes readonly package version" lib/bash/std/tests/lib_std.bats failed before the fix because BASE_BASH_LIBS_VERSION was unset.
• GREEN: bats --filter "stdlib exposes readonly package version" lib/bash/std/tests/lib_std.bats
• bats lib/bash/std/tests/lib_std.bats
• shellcheck --severity=error lib/bash/std/lib_std.sh lib/bash/std/tests/lib_std.bats
• git diff --check
• ./tests/validate.sh

Fixes #16