Rework TestExplorer context to reducer model

[veebs]

Rework TestExplorer context to reducer model:

• not use writable (Svelte store)
• reducer should return new state instance

Check FileExplorer code

[quockhanh]

Lowest priority please, do the main feature first

[quockhanh]

Should be done after ticket #40 . The reason is:

• If we dont use selected: writable<boolean>, we have to find Node (by tree path) to update selected: boolean manually
• However, there is a test case which name is sub-path/my-test-case.tcase. We cannot handle this case right now becuase findNodeBypath will lookup a file my-test-case.tcase under sub-path folder, which does not exist
• Ticket #40 requires to display sub-path folder and my-test-case.tcase file as separated nodes, then we can use above approach