FEP-5e53 : add fancy @Context file for new terms added by FEP-5e53 (#348)

I barely have the faintest idea how `@Context` files work, but I think this way of declaring new terms helps them be merged into others?  If not, please advise LD-wizards 🧙

Recreates the accidentally-deleted branch from [earlier PR 329](https://codeberg.org/fediverse/fep/pulls/329/files).

Not sure how to tag @a or @dmarti for review but at least I can link this from the tracking issue #327 ?

Co-authored-by: bumblefudge <bumblefudge@learningproof.xyz>
Reviewed-on: https://codeberg.org/fediverse/fep/pulls/348
Co-authored-by: bumblefudge <bumblefudge@noreply.codeberg.org>
Co-committed-by: bumblefudge <bumblefudge@noreply.codeberg.org>

fep/5e53/context.jsonld

@@ -0,0 +1,14 @@
+{
+  "@context": {
+    "xRobotsTag": {
+      "@id": "https://www.robotstxt.org/meta.html",
+      "@type": "@id",
+      "@container": "@list"
+    },
+    "SPC": {
+      "@id": "https://privacycg.github.io/gpc-spec/#the-sec-gpc-header-field-for-http-requests",
+      "@type": "@id",
+      "@container": "@list"
+    }
+  }
+}