fix(runtime): append start plugins to rtp before calling preload hooks

[mrcjkb]

Closes #237.

This fixes the loading order issue (e.g. colorschemes) for rocks-config.nvim:

1. Append start plugins to the rtp
2. Call preload hooks -> rocks-config.nvim
3. Source plugin directories

With this, I don't think rocks-config needs to worry about providing before or after config mechanisms.

[github-actions[bot]]

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