fix: Retrieve diagnostics for changed document only
fix: Retrieve diagnostics for changed document only

Currently, when a document is changed, we retrieve diagnostics for
**all** open files. This could be very slow when there are many open
files. Instead, retrieve diagnostics for that one file only.

angular/vscode-ng-language-service,angular/vscode-ng-language-service,angular/vscode-ng-language-service,angular/vscode-ng-language-service