build: add bazel config for cdk-experimental/dialog

[crisbeto]

Adds the BUILD.bazel file for the @angular/cdk-experimental/dialog package for consistency with everything else and to make it easier to eventually move it into the CDK.

[crisbeto]

A note on this one: I still can't find a way to get Bazel to pick up the typings.d.ts that is one level up in cdk-experimental. This is also something that came up in #11481. Changing it in the tsconfig works when running it through Gulp, but not through Bazel.

[jelbourn]

That's expected; Bazel doesn't allow referencing source files that aren't in the package's subtree; if you wanted to reference a common d.ts file, you have to create a build rule for it and depend on that.

[jelbourn]

LGTM

[josephperrott]

@crisbeto Please rebase when you can

[crisbeto]

Rebased.

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