Implement SheetHolder and cross-sheet referencing

[Fluglow]

Closes #101

SheetHolder

• Implement SheetHolder as a singleton stored in the window object
• Only track Lightsheet objects in SheetHolder -> don't allow Sheet objects to exist without Lightsheet
  • SheetHolder would otherwise need to track both Lightsheet objects and "unattached" Sheet objects

Sheet / ExpressionHandler

• Add expression parsing for referring to other Sheets' cells (=SheetName!A1)
• Add SheetKey and add it to types used in expression parsing and cell reference handling
• Update cell updating and reference handling logic to account for cells in different sheets
  • When a change happens in a Sheet, that Sheet is responsible for updating all other Sheet objects
• Move cellData to SheetHolder
  • This simplifies lookups with multiple sheets since CellKeys are independent

Lightsheet

• Add sheet name to LightsheetOptions, default null data to []
• Allow Lightsheet to be instantiated without UI

Refactoring

• Make type { columnPosition, rowPosition } shared as Coordinate in utils/common.types.ts
  • Should this file be somewhere else / called something else?
• Move utils folder under src

[Yousif-FJ]

Here is my opinion on this and why I think this is problematic:

sheet holder containing cellData

• Sheet containing SheetHolder when the hierarchy should be the other way around.
• SheetHolder holding cellData when we expect it to hold Sheet.
• SheetHolder holding LightSheet when we expect to hold Sheet.
• Extreme coupling between Sheet and SheetHolder while SheetHolder should only be necessary when evaluating expression.

My suggestion, would be to go back to something similar to the structure we agreed on:

• sheetHolder holding sheet
• We create a new class called EvaluationContext which a LightSheet level object that encapsulate expressionHandler and the logic of evaluating math, and contains sheetHolder
• Refactor resolve cell to do reference handling from the new class.

I know this might be a big change from what you did, but I have to give my opinion.

[Fluglow]

@Yousif-FJ I think your proposals should be implementable from this. I chose this easier route since separating logic from Sheet proved to be quite tricky, but it should be easier from this stage. The main issue was that we want to keep most methods private in Sheet, and separating logic easily leads to having to call these internal methods from elsewhere.

SheetHolder containing cellData is akin to the structure of having a separate DataManager, but they are just combined now. I again see the benefit of holding cellData separate from Sheet, since cell information doesn't need to be indexed by anything else (holding it in Sheet is essentially indexing by SheetKey). Cell objects are also cross-sheet by nature as they can hold keys from multiple different sheets, so having them in a broader context seems more correct.

[rafinutshaw]

This excellent work @Fluglow

[rafinutshaw]

When i try to refer to a sheet that does not exist. it show show an error just like in excel. instead it is clearing the field