LSP: fetch the file content from cache instead of from disk to compute the line number

Otherwise it doesn't work with the web extension. And the file must anyway be in the cache

internal/compiler/diagnostics.rs

@@ -104,6 +104,10 @@ impl SourceFileInner {
                 .unwrap_or_default()
         })
     }
+
+    pub fn source(&self) -> Option<&str> {
+        self.source.as_ref().map(|s| s.as_str())
+    }
 }
 
 pub type SourceFile = Rc<SourceFileInner>;