Fix Node.js backtick imports and README commands

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Summary

• Match Node.js require/import calls that use backtick string literals in GuessRegexps.
• Add regression coverage for backtick require/import matching.
• Make Node.js guess tests exercise the temp project directory and close temp files correctly on Windows.
• Update Node.js guess expectations for files where the current parser still finds require/import calls after TypeScript type declarations.
• Remove shell prompt prefixes from README command examples so copied commands run directly.
• Add source build instructions to the README.

Fixes #405.
Addresses #363.
Addresses the source-build-instructions portion of #262.

Testing

• git diff --check
• go test ./internal/backends/nodejs