Build system updates

[wch]

This PR makes many updates to the build system. Some of these changes are in preparation for future changes to the extension.

• Switch from yarn to npm for simplicity (and because npm is much better these days than it was in the past)
• Use esbuild for building - this is important for future work which requires a more sophisticated build system
• Install prettier, auto-sort imports, add prettier as a recommended VS Code extension, and enable auto-formatting on save
• Reformat files with prettier
• Update eslint to version 9, and base the configuration on the eslint recommended settings
• Fix issues flagged by eslint
• Update the copyright holder from RStudio to Posit