feat: provide tabs and template to scratchpad for side buffers

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Describe the problem

submitted by @premell

see 3. and 4. in #207.

Describe the solution

3. a source selector like neotree offers could provide more flexibility on the content displayed in the side buffer, as for example switching between many scratchpad files etc.

4. it would be great to have base templates for the scratchpad to be pre-filled with, not yet sure how it would be provided by the user, but maybe via a "base file path" so it reads it and fill the buffer with it.

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#303