Component Slots

[ubugeeei]

related: #4

[ubugeeei]

I tried implementing slots, but there seem to be some areas that likely require discussion.
I've created a draft PR, but this is a provisional implementation.
I will compile the points that need to be discussed in this issue soon.

[sxzz]

Big thanks for awesome your contribution! However, we need to slow down due to my working schedule.

[ubugeeei]

Ok, I'll wrap up what I've done so far with this issue's note and take a break. 😴
If there's anything low-effort you'd like me to do first, please don't hesitate to ask. (Like adding small tests, etc.)

[sxzz]

Continues in #154