Add esbonio.sphinx.buildTriggers config option

[alcarney]

This option controls when the language server rebuilds the project. The server's default configuration is equivalent to setting

      {
        "esbonio.sphinx.buildTriggers": {
          "onSave": true,
          "onChange": 2.0, // seconds
        }
      }

where it will rebuild each time a file is saved, or each time a file is changed - with a 2 second delay. Setting onChange to false will disable it entirely.

Closes #935