[TVMScript] Printer VarTable

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This PR:

• Adds VarTable for the new TVMScript Printer

Compared to the prototype version, this:

• Removes unnecessary public methods.
  • GetObjectName
  • GetUniqueName
• Changes DocFactory from ExprDoc(ObjectPath) to ExprDoc() to simplify var definition.
• Removes the dependency on TracedObject to be FFI friendly

It's in draft state because it's branched off from a pending PR #12237

Tracking issue: https://github.com/apache/tvm/issues/11912

cc @junrushao1994 @gbonik