Ignore TODO in pkg/analyzer_plugin.

The analyzer team has 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.  They may even consider adding a presubmit hook
to ensure that `FIXME` comments are only used during local
development.

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

(See also https://dart-review.googlesource.com/c/sdk/+/295662, which
made the corresponding change to the `analysis_server` and `analyzer`
packages, and https://dart-review.googlesource.com/c/sdk/+/325121,
which made the corresponding change to the `linter` package).

Change-Id: Ifbd2f6bed468408b1906b25ab94f745eed0b3e55
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/358980
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>
Commit-Queue: Paul Berry <paulberry@google.com>

pkg/analyzer_plugin/analysis_options.yaml

@@ -4,6 +4,7 @@ analyzer:
   language:
     strict-casts: true
   errors:
+    todo: ignore
     # Remove from lints/recommended:
     constant_identifier_names: ignore
     implementation_imports: ignore