Ignore TODOs in analyzer_utilities package.

Some time ago, the analyzer team decided to adopt the convention of
using `TODO` comments to document long term issues that should persist
in the codebase, and `FIXME` comments to document short term issues
that need immediate attention.

Accordingly, it makes sense to suppress `TODO` comments from being
surfaced to the IDE "problems" view (since there can be a lot of them,
and they're not immediately actionable).  This makes VSCode's
"problems" view much more usable in "tree" mode.

Previous CLs that have made this change in other packages:
- https://dart-review.googlesource.com/c/sdk/+/295662
- https://dart-review.googlesource.com/c/sdk/+/325121
- https://dart-review.googlesource.com/c/sdk/+/358980
- https://dart-review.googlesource.com/c/sdk/+/410060
- https://dart-review.googlesource.com/c/sdk/+/440546
- https://dart-review.googlesource.com/c/sdk/+/442167

Change-Id: I06c0118c7b166a6fad550e8f280f3732734a74e1
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/443633
Auto-Submit: Paul Berry <[email protected]>
Reviewed-by: Konstantin Shcheglov <[email protected]>
Commit-Queue: Konstantin Shcheglov <[email protected]>

Author: stereotype441

pkg/analyzer_utilities/analysis_options.yaml

@@ -5,6 +5,10 @@ analyzer:
     strict-casts: true
     strict-inference: true
     strict-raw-types: true
+  errors:
+    # Ignoring this for all developers means developers don't need to ignore
+    # TODOs in their IDE settings.
+    todo: ignore
 
 linter:
   rules: