User can debug their Unity game with dnspy and other debugger, it needn't patch mono.dll.
most code is copied&modified from [UnityDoorstop](https://github.com/NeighTools/UnityDoorstop/blob/master/src/bootstrap.c#L328)
currently it work on SDEZ/SDDT
<details>
<summary>How to debug and breakpoint Unity game (For example SDDT)</summary>
1. Modify segatools.ini and set `enableDebug` and `debugAddress`

2. Inject mu3 with hook dll

3. dnspy connect unity

4. open Modules tab and double click Assembly-CSharp.dll, yeah, now you can add breakpoints now.

</details>
and make unity_config.target_assembly is optional if we just want to debug unity.
Now PR is ready to code review and merge into upstream.
Reviewed-on: https://gitea.tendokyu.moe/TeamTofuShop/segatools/pulls/99
Co-authored-by: MikiraSora <mikirasora0409@126.com>
Co-committed-by: MikiraSora <mikirasora0409@126.com>