[Snapshot] Add BUILD file. (#6308)
[Snapshot] Add BUILD file. (#6308)

Creating a BUILD file and public "Snapshot" target for clients to depend on.
The primary use of this target will be in material_components_ios.bzl to
generate snapshot build rules.

Closes #6230
material-components/material-components-ios,material-components/material-components-ios,material-components/material-components-ios,material-components/material-components-ios,material-components/material-components-ios