Ignore TODOs in analysis_server_plugin.

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.

(See also https://dart-review.googlesource.com/c/sdk/+/295662, which
made the corresponding change to the `analysis_server` and `analyzer`
packages, https://dart-review.googlesource.com/c/sdk/+/325121, which
made the corresponding change to the `linter` package, and
https://dart-review.googlesource.com/c/sdk/+/358980, which made the
corresponding change to the `analyzer_plugin` package).

Change-Id: I916c8c7f435d06c1b0bd871ef3829d58c9242218
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/410060
Reviewed-by: Samuel Rawlins <[email protected]>
Auto-Submit: Paul Berry <[email protected]>
Commit-Queue: Paul Berry <[email protected]>

Author: stereotype441

pkg/analysis_server_plugin/analysis_options.yaml

@@ -12,6 +12,10 @@ analyzer:
     implementation_imports: ignore
     non_constant_identifier_names: ignore
 
+    # Ignoring this for all developers means developers don't need to ignore
+    # TODOs in their IDE settings.
+    todo: ignore
+
 linter:
   rules:
     - always_use_package_imports