specs: import OSC 3008 spec from systemd (#202)

This stuff is really generic, and fits the purpose of the UAPI group,
hence let's migrate this here.

This is mostly a 1:1 copy of the spec from systemd, but I enclosed the
final part that carries a list of OSC sequences currently known in
markdown comments, because it's not really part of the spec, but still
kinda useful for spec writers.

(Maybe one day we should turn that table into a "registry" style spec of
its own?)

Author: poettering

README.md

@@ -51,6 +51,9 @@ The following specifications have been accepted by the UAPI group:
 * [UAPI.14 Virtual Machine Generation ID](specs/vmgenid.md):
   Describes the mechanism for detecting virtual machine rollback events.
   ([canonical online location](https://uapi-group.org/specifications/specs/vmgenid/))
+* [UAPI.15 OSC 3008: Hierarchical Context Signalling](specs/osc_context.md):
+  Defines a mechanism for terminal emulators to follow the context hierarchy of what's on screen.
+  ([canonical online location](https://uapi-group.org/specifications/specs/osc_context/))
 
 ## Work in Progress