feat(builtin): add a DeclarationInfo provider

This gives us a principled way for rules to generate and consume TypeScript typings

Author: alexeagle

declaration_provider.bzl

@@ -0,0 +1,28 @@
+"""This module contains a provider for TypeScript typings files (.d.ts)"""
+
+def provide_declarations(**kwargs):
+    """Factory function for creating checked declarations with externs.
+
+    Do not directly construct DeclarationInfo()
+    """
+
+    # TODO: add some checking actions to ensure the declarations are well-formed
+    return DeclarationInfo(**kwargs)
+
+DeclarationInfo = provider(
+    doc = """The DeclarationInfo provider allows JS rules to communicate typing information.
+TypeScript's .d.ts files are used as the interop format for describing types.
+
+Do not create DeclarationInfo instances directly, instead use the provide_declarations factory function.
+
+TODO(alexeagle): The ts_library#deps attribute should require that this provider is attached.
+
+Note: historically this was a subset of the string-typed "typescript" provider.
+""",
+    # TODO: if we ever enable --declarationMap we will have .d.ts.map files too
+    fields = {
+        "declarations": "A depset of .d.ts files produced by this rule",
+        "transitive_declarations": """A depset of .d.ts files produced by this rule and all its transitive dependencies.
+This prevents needing an aspect in rules that consume the typings, which improves performance.""",
+    },
+)