Move spx+ from `goplus/spx` to `goplus/builder`

[nighca]

There are two versions of "spx":

• the game engine spx
• the game runtime "spx+" for XBuilder

"spx+" includes the game engine and additional packages like ai.

Currently, the "spx+" implementation is in the goplus/spx repository. To clarify responsibilities, it should be moved to goplus/builder.

For example, in web releases, goplus/spx provides a runner page with a base gdspx.wasm (containing only the game engine). goplus/builder should build its own gdspx.wasm representing "spx+" and use it in the runner page instead.

[nighca]

Related: https://github.com/goplus/spx/pull/966 (Now the "spx+" implementation for PC platform is also maintained in repo goplus/spx)