Programmatically enable linked editing range

[datho7561]

If we enable linked editing range for all xml documents by contributing the following setting:

"[xml]": {
  "editor.linkedEditing": true,
},

Then it will override whatever the user sets. However, if we enable the setting through the VS Code API in TypeScript, we can first check if the user has explicitly enabled/disabled it first, so that we don't override what the user has set.