feat: provide some state and methods via slot scope

[mesqueeb]

@tochoromero this is still a WIP. I have yet to test these changes.

I think it's relatively safe since VTh will always be a direct child to VTable and therefore can be considered tightly coupled. So this is a rare case where it's safe to access parent methods & state imo.

I have added an extra check in the setup to check for the case where VTh is for some reason not a child of VTable.

I will continue to test and update this PR.

[mesqueeb]

@tochoromero

Check out my branch and run npm run dev.
Then check out the source code for my proof of concept here:
https://github.com/tochoromero/vuejs-smart-table/blob/4c25e76b7662cda692e7c7f07bcb73e9a902c97e/App.vue#L14~L20

See the GIF here:

Some notes:

• I added a new prop called canSort for the VTh component, to make it possible to use VTh for other things besides sorting as well. Otherwise it would show a sortable icon by default.
• I made canSort true by default, in order to have 0 breaking changes.
• I felt it's cleaner to add a prop called canSort as opposed to not adding any new props but doing something like :sortKey="false" instead. Let me know what you think.

Please tell me if you are OK with this implementation. I will continue the PR appropriately.
If OK to proceed I will...

• For VTh: create a computed property called allSelected (with get & set syntax) like described in my issue before, where setting this to true will execute selectAll() so it can be linked to a checkbox.
• For VTr: make similar changes to the VTr component but call it isSelected instead

What do you want me to do with the dev playground file? Shall I delete those changes again before finalising the PR?

[mesqueeb]

@tochoromero did you see this PR? You've addressed what I tried to achieve with this PR in the latest release, so should I go ahead and close this PR?

PS: a final question for you.
I did research how to best access the Vue parent component instance in Vue 3 and for tightly coupled components came up with this:

    const internalInstance = getCurrentInstance()
     const parentInstance = internalInstance?.parent?.proxy
     if (!parentInstance || parentInstance?.$options.name !== 'VTable') {
       throw new Error('VTh must be used in the header slot of VTable.')
     }

then further down I was passing parts of the parent scope to the slot scope like so:

 children.push(h('span', [slots.default({
           sortOrder: order.value,
           rows: (parentInstance as any).tableState.rows,
           selectedRows: (parentInstance as any).tableState.selectedRows,
           selectAll: () => (parentInstance as any).selectAll(),
           deselectAll: () => (parentInstance as any).deselectAll(),
         })]))

I was hoping to get your thoughts on this, because you used a very different mechanic to achieve the same.