removed csproj-based path config and dynamically derive project root for SerializationInfo generation

- Removed dependency on csproj MSBuild properties for setting MemoryPackGenerator_SerializationInfoOutputDirectory.
- Dynamically derive the project root by locating the "Assets" folder in source file paths and using its parent directory.
- Generate SerializationInfo for all files marked with [MemoryPackable].

Update MemoryPackGenerator.cs to always generate SerializationInfo files

Resolve the serialization info output folder from each type's own source file
instead of once per compilation: walk up to the nearest ancestor holding a .git
entry and write to <repo root>/MemoryPackSerializationInfo~ there, creating it
when missing. Types declared in a submodule now dump into that submodule, so a
serialized layout change shows up in the diff of the pull request that made it.

The '~' suffix keeps Unity's asset pipeline out of the folder (no TextAsset
import, no .meta files, no import triggered by writing during compilation).
Files outside any git repository are skipped instead of landing on the Desktop.

Removes the Collect() over every [MemoryPackable] syntax node, which used to
invalidate all downstream generation on any edit to one of those files.

This reverts commit 0170c59.

This reverts commit f6bcc09.