Add support for feature order definition in devcontainer definitions

Currently, the order that features are executed as part of the container build is alphabetically sorted in the following code, where there is a TODO to implement support for the "installation order" that is part of the devcontainer spec: https://containers.dev/implementors/features/#installation-order

https://github.com/coder/envbuilder/blob/c72728897e91e713958a8ca0c7211574784ae79e/devcontainer/devcontainer.go#L236-L244

The ask in this issue is to implement support for:

> - The `dependsOn` property defined as a part of a Feature’s devcontainer-feature.json.
> - The `installsAfter` property defined as part of a Feature’s devcontainer-feature.json.
> - The `overrideFeatureInstallOrder` property in user’s devcontainer.json. Allows users to control the order of execution of their Features.

I wasn't able to find an existing issue tracking this, but let me know if there is! Happy to also provide more context on why this is a useful feature if that helps in prioritizing this. Features with any sort of dependencies are unusable in most cases unless the alphabetical ordering happens to line up.

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In the meantime, it would be helpful to have this documented in the way that this PR did for other unsupported features: https://github.com/coder/envbuilder/pull/219

Start in devcontainer/devcontainer.go at the linked feature-ordering code and read the devcontainer specification's installation-order section. Trace how feature metadata from devcontainer-feature.json and configuration from devcontainer.json are loaded. Done means dependsOn, installsAfter, and overrideFeatureInstallOrder affect execution order, with unsupported behavior documented as shown by PR #219.

Vom Indexierungsmodell aus dem Issue-Text verfasst.