/gemini summary

Summary of Changes

I've introduced shellcheck to our dataconnect bash scripts to improve their quality and maintainability. This PR adds a new shellcheck.sh script that lints all relevant bash files and applies various syntax improvements across existing scripts to adhere to shellcheck recommendations.

Highlights

• New Shellcheck Script: A new script, firebase-dataconnect/scripts/shellcheck.sh, has been added to automate the linting process for all bash scripts within the emulator and scripts directories.
• Bash Script Modernization: Several existing bash scripts have been updated to improve syntax and robustness, including changes to readonly variable declarations and consistent use of curly braces for variable expansions.
• Improved Path Handling: The method for defining SCRIPT_DIR and PROJECT_ROOT_DIR in multiple scripts has been modified, separating the assignment from the readonly declaration for better clarity and adherence to linting rules.
• Array Element Access Correction: Array element access in emulator.sh was corrected from "${gradle_emulator_binaries[@]}" to "${gradle_emulator_binaries[0]}" for more precise single-element retrieval.