vscode: declare contributes.breakpoints for sx (enable breakpoint gutter)

VSCode disables the breakpoint gutter for a language unless an extension
declares breakpoints are valid for it. The sx extension registered the
language but never contributed breakpoints, so clicking the gutter in a
.sx file did nothing. Add the breakpoints contribution so users can set
breakpoints in .sx files without the per-workspace
debug.allowBreakpointsEverywhere hack.

editors/vscode/package.json

@@ -27,6 +27,11 @@
         "configuration": "./language-configuration.json"
       }
     ],
+    "breakpoints": [
+      {
+        "language": "sx"
+      }
+    ],
     "grammars": [
       {
         "language": "sx",