Performance and Code Improvements

[SujitSingh]

This PR is for improvements added for PNG generation feature.

Changes summary -

• Using sharp for image export operations. Removed dependencies of svgexport, imagemin-cli and exec checks.
• Removed necessity of temp-image generation
• Code readability improvements. Though new changes have cameCase variable naming instead of snake_case.
• Added sharp under dependencies of package.json
• Using "type": "module" in package.json for using import rather than require.
• Improvements to .gitignore file.
• README file update.

Results -

• Improved performance -> I think it is more than 10x fast but I haven't compared them using benchmarks.
• Simpler and more readable code.
• Single command for installing dependencies using just npm install.