Improve dialog string variables

[Ramza13]

Summary

None

Purpose of change

Part of #53625
Fixes #55155
Add debug commands to set variables

Describe the solution

Add new functions to use variable access syntax to allow string variables of u, npc and global to be changed/tested.
Add tag parsing to message function so variables can be added to messages.
Shamelessly stole the str_or_var code from #55233

Describe alternatives you've considered

Testing

Used this json:

{
    "type": "effect_on_condition",
    "id": "test",
    "condition": { "compare_string":[{"global_val":"test_var", "default_str":"race"},"Test"] },
    "effect": [{ "u_message": "<global_val:tester> testtastic" },{"set_string_var":"fun", "target_var":{"global_val":"tester"}}]
  },

Additional context