Coxbuild

Move extension gallery to [gallery repo](https://github.com/StardustDL/coxbuild-ext-gallery).

Add extension gallery based on jsDelivr CDN `ext://` and GitHub raw `extraw://`.

- Add full extension implementation
- Extension class and loader
- Extension URI `src://`, `url://`, `file://`, `module://`
- Treat schema as an extension.
- Support extension URI as schema option's value.

- Use task as event handler.
- Update TaskContext, PipelineContext, ServiceContext, ExecutionState.
- Support multiple pipeline/service execution.
- Remove shared global variables for configuration data.
- Transmit configuration and execution state in runtime arguments.
- A series of helper decorators `with[Argument]`

- Support multiple hooks.
- Simplify decorators: `task()` -> `task`.
- Support **forwards** and **backwards** task configuration.

python
task1.depend
task
def pre(): pass

or

depend(pre)
task
def task1(): pass

- Update documents and unit-tests.
- Fix tiny bugs detected by unit-test.