JS: Model fs.promises.readFile as file source

You could argue that proper modeling be done in the same way as
`NodeJSFileSystemAccessRead` is done for the callback based `fs` API (in
NodeJSLib.qll). However, that work is straying from the core goals I'm
working towards right now, so I'll argue that "perfect is the enemy of
good", and leave this as is for now.

javascript/ql/lib/semmle/javascript/frameworks/NodeJSLib.model.yml

@@ -0,0 +1,8 @@
+extensions:
+  # Make sure that the extensible model predicates have at least one definition
+  # to avoid errors about undefined extensionals.
+  - addsTo:
+      pack: codeql/javascript-all
+      extensible: sourceModel
+    data:
+      - ['fs', 'Member[promises].Member[readFile].ReturnValue.Member[then].Argument[0].Parameter[0]', 'file']

javascript/ql/test/library-tests/threat-models/sources/sources.js

@@ -69,8 +69,8 @@ const fileContent = fs.readFileSync('file.txt', 'utf8'); // $ threat-source=file
 SINK(fileContent); // $ hasFlow
 
 // Accessing file contents using fs.promises
-fs.promises.readFile('file.txt', 'utf8').then((data) => { // $ MISSING: threat-source=file
-  SINK(data); // $ MISSING: hasFlow
+fs.promises.readFile('file.txt', 'utf8').then((data) => { // $ threat-source=file
+  SINK(data); // $ hasFlow
 });
 
 // Accessing file contents using fs.createReadStream