Implement TS typings

[areknawo]

This PR adds TS typings with the following code changes:

• reduce the use of undefined in favor of null (consistency & easier typing)
• use multiple parameters instead of objects where appropriate (easier typing)
• use .ts extensions where TSX is not required
• use single export at the bottom of the file (consistency, readability)
• rename DragDropContext and SortableContext to DragDropProvider and SortableProvider respectively (in-line with Solid.js naming practices)

⚠️ Changes in naming are breaking.

[martinpengellyphillips]

Great to see this @areknawo. I see you are still adding a few commits - let me know when you want me to do a first pass review and I'll jump in.

[areknawo]

I've done some tests with all the examples from the docs and it's all good now. I think you can do the first pass now.

[martinpengellyphillips]

Looking good - thanks again for the work here. I've tested with a few of my sites now and only found a few issues (see comments on files).

I'll give you some time to respond, but if you are busy just let me know - I can always merge and then make the decisions/changes myself before release.

Notes to self:

• We should add you as a contributor somewhere - perhaps in the README or a CONTRIBUTORS.md file.
• Update README.md to reflect changes.
• Update CHANGELOG with changes. In particular, highlight breaking changes of renames and the changes to function parameters style.
• Fix min SolidJS version compatibility to be >= 1.0.5

[martinpengellyphillips]

Okay - I think everything is done (including CHANGELOG and README updates).

I'll do one more test later today with my repos and then merge and release by end of this week. Thank you again for all the help!!