Update TS definition for Filters plugin and Core methods

[budnix]

Context

The PR updates the wrong TS definition for the Filters plugin methods and the definition for returned values of the colToProp method. The PR moves tests from general files to individual ones in the plugins directory.

How has this been tested?

I tested the changes locally and I covered the fix with types tests.

Types of changes

• Bug fix (non-breaking change which fixes an issue)

Related issue(s):

1. fixes https://github.com/handsontable/dev-handsontable/issues/446

Affected project(s):

• handsontable

Checklist:

• I have reviewed the guidelines about Contributing to Handsontable and I confirm that my code follows the code style of this project.
• I have signed the Contributor License Agreement