insert and delete variables are expanded with `@base` `@vocab` contexts

[dpetran]

When a transaction has a context that defines @base and @vocab, variables within the the insert and delete clauses get erroneously expanded:

(json-ld/expand {"@context" {"@base" "ex:ns/" "@vocab" ""}
                 "@id" "?s"
                 "foo" "bar"})
;;=>
{:idx [], :id "ex:ns/?s", "ex:ns/foo" [{:value "bar", :type nil, :idx ["foo"]}]}
;; note the "ex:ns/?s" id instead of just "?s"

We need to avoid expanding these variables so they can be parsed correctly.

Here are three possible approaches:

1. make json-ld/expand aware of variables so it can skip expanding them
2. make the variables valid iris so they won't get captured by @base
3. don't json-ld/expand at all, manually traverse the tree and call expand-iri where we want it (like how we do where-clause parsing)

json-ld/expand cannot handle variable symbols either, with the same possible solutions.

[dpetran]

After discussing with Ben, we're going to go with option 3.

[aaj3f]

Relevant comments & examples provided by @Jackamus29 in this thread: https://fluree-internal.slack.com/archives/C04AA19P3FW/p1706296205125759?thread_ts=1706294773.772209&cid=C04AA19P3FW

[Jackamus29]

Here's another case may be fixed when this ticket is resolved:

Description delete+where doesn't work if @vocab is present.

Steps to Reproduce

1. Create ledger with data

   {
   "ledger": "vocab-test",
   "@context": {
   "@vocab": "https://flur.ee/"
   },
   "insert": [
   {
     "@id": "freddy",
     "@type": "Yeti",
     "name": "Freddy"
   }
   ]
   }

2. Delete subject

   {
   "ledger": "vocab-test",
   "@context": {
   "@vocab": "https://flur.ee/"
   },
   "where": {
   "@id": "freddy",
   "?p": "?o"
   },
   "delete": {
   "@id": "freddy",
   "?p": "?o"
   }
   }

3. History query shows no retractions

   {
   "from": "vocab-test",
   "commit-details": true,
   "t": {
   "at": "latest"
   },
   "@context": {
   "@vocab": "https://flur.ee/",
   "f": "https://ns.flur.ee/ledger#"
   }
   }

   and query shows the subject

   {
   "from": "vocab-test",
   "select": {
   "freddy": [
     "*"
   ]
   }
   }

   =>

   [
   {
   "@id": "freddy",
   "@type": "https://flur.ee/Yeti",
   "https://flur.ee/name": "Freddy"
   }
   ]