feature: Update builds to use STDLIB_BRANCH environment variable when set

[duaneatat]

Part of #101

Checklist

• Modify tools/scripts/utils/stdlib_version.js to return STDLIB_DOCS_DIRNAME env var if set
• Add STDLIB_BRANCH variable to Makefile (defaults to develop)
• Add logic to compute STDLIB_DOCS_DIRNAME from STDLIB_BRANCH:
  • develop → latest
  • Other branches: sanitize by replacing / with - (e.g., feature/docs → feature-docs)
• Create clone-stdlib-version target that:
  • Removes existing node_modules/@stdlib/stdlib
  • Clones specified branch/tag into node_modules/@stdlib/stdlib
  • Defaults to develop branch when STDLIB_BRANCH is not specified
  • Runs make install-node-modules in stdlib directory
• Modify api-docs target to:
  • Depend on clone-stdlib-version
  • Pass STDLIB_DOCS_DIRNAME to build scripts
• Test make api-docs uses develop branch (outputs to latest/)
• Test make api-docs STDLIB_BRANCH=v0.2.0 (outputs to v0.2.0/)
• Test with branch containing / (e.g., feature/docs → feature-docs/)
• Verify STDLIB_DOCS_DIRNAME is set correctly for all build scripts
• Verify multiple stdlib docs versions load (with an uncommitted change to config/index.js

Description

This pull request implements the first step in #101

1. Make build system version-aware

Make build scripts respect STDLIB_VERSION environment variable and output to version-specific directories (e.g., public/docs/api/v0.2.0/). When building, always clone stdlib into node_modules/@stdlib/stdlib - defaulting to develop branch for latest, or the specified version/tag otherwise.

So now you can build the docs for v0.3.2 with make build STDLIB_BRANCH=v0.3.2 and the generated docs will end up at public/docs/api/v0.3.2.

@stdlib-js/reviewers