Refactor SQL Preview webview implementation

[RandomFractals]

• Create standard src/views folder
• Move src/sql_output/index.ts to `src/views/sqlPreview.ts
• Create SqlPreview webview class in sqlPreview.ts instead of using only functions to create that webview panel as we do now

This would make Sql Preview code much more readable, more OOP, and match typical webview setup other vscode extensions use.

See this tableView.ts for an example of a well-designed webview implementation.