terminal/osc: don't export context/semantic prompts to libvt yet

src/terminal/osc/parsers/context_signal.zig

@@ -16,6 +16,8 @@ const max_context_id_len = 64;
 
 /// A single OSC 3008 context signal command.
 pub const Command = struct {
+    pub const C = void;
+
     action: Action,
     /// The context identifier. Must be 1-64 characters in the 32..126 byte range.
     id: []const u8,

src/terminal/osc/parsers/semantic_prompt.zig

@@ -14,6 +14,8 @@ const log = std.log.scoped(.osc_semantic_prompt);
 /// all except one do and the spec does also say to ignore unknown
 /// options. So, I think this is a fair interpretation.
 pub const Command = struct {
+    pub const C = void;
+
     action: Action,
     options_unvalidated: []const u8,