feat: Expose InitAssets to allow projects initialise assets to another directory

[junghao]

Proposed Changes

Changes proposed in this pull request:

• Expose weft.InitAssets() so the using project can override the default assets/assets directory. This is necessary for Ko based build (Ko uses kodata as asset directory and cannot be changed).

The default init() still works so no impact to any existing projects. This is just opening a window for projects which need this. For projects using Ko, run InitAssets(kodatapath+"/assets", "assets") before mux init.

Production Changes

The following production changes are required to deploy these changes:

• None

Review

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[bpeng]

OK