Ensure appropriate code review of changes to the analyzer public API.

This change uses Gerrit OWNERS file syntax (documented here:
https://gerrit.googlesource.com/plugins/find-owners/+/HEAD/src/main/resources/Documentation/syntax.md?pli=1)
to ensure that any changes affecting pkg/analyzer/api.txt will be
reviewed by the developer experience team (or one of the last resort
global reviewers), rather than the Dart Model team.

Since changes to the analyzer that affect its public API will
inevitably affect both .dart files in pkg/analyzer (or
pkg/_fe_analyzer_shared) as well as pkg/analyzer/api.txt, this
ownership change ensures that any change affecting the analyzer public
API will be reviewed both by a member of the Dart Model team and by a
member of the developer experience team.

Change-Id: I07527efe1fa7aa3f1d75488c1800410c2aaaa6ec
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/410660
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Reviewed-by: Johnni Winther <johnniwinther@google.com>
Commit-Queue: Paul Berry <paulberry@google.com>

pkg/analyzer/OWNERS

@@ -1 +1,8 @@
 file:/tools/OWNERS_MODEL
+
+# api.txt is a generated file that documents the types and methods in
+# the analyzer public API; accordingly, changes to it need to be
+# reviewed by the developer experience team.
+per-file api.txt=set noparent
+per-file api.txt=file:/tools/OWNERS_DEVELOPER_EXPERIENCE
+per-file api.txt=file:/OWNERS # (For last resort global approvers)