Develop and Register Custom Functions

## Read and complete the full issue template

**Do you want to request a *feature* or report a *bug*?**
- [] Bug
- [x] Feature

If you are logging a possible bug or feature request, please test with [the latest development build](https://ci.appveyor.com/project/ClosedXML/ClosedXML/branch/develop/artifacts) first.

**Version of ClosedXML**

v0.94.2

**What is the current behavior?**

If an Excel workbook uses custom VBA functions, ClosedXML will fail to parse the formulas because it cannot interpret the custom function

**What is the expected behavior or new feature?**

I was wondering if you guys ever considered supporting custom functions. At very high level, it could be achieved by making certain classes public or creating new interfaces (CalcEngine, CalcEngineFunction, Expression, etc), with added validations and logic.

I want to discuss this before spending time developing it.

The issue identifies CalcEngine, CalcEngineFunction, and Expression as possible extension points, but names no files or tests. Start by locating those classes and the formula-parsing path, then clarify the supported custom-function API and validations; completion would require an agreed scope and a demonstrable way to parse workbooks containing custom VBA functions.

Written by the indexing model from the issue text.