Add preprocessor directives #define and #undef

I propose we add `#define` and `#undef` preprocessor directives for conditional compilation (as in C#: https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/preprocessor-directives#defining-symbols).

## Pros and Cons

The advantages of making this adjustment to F# are
1. Good for quick debugging and code maintenance
2. No ground-breaking change to the language

Honestly, I don't know if there's any disadvantage.

## Extra information

Estimated cost (XS, S, M, L, XL, XXL): S

Related suggestions: I was not able to find relevant issues.

## Affidavit (please submit!)

Please tick this by placing a cross in the box:
* [x] This is not a question (e.g. like one you might ask on [stackoverflow](http://stackoverflow.com)) and I have searched stackoverflow for discussions of this issue
* [x] I have [searched both open and closed suggestions on this site](http://github.com/fsharp/fslang-suggestions/issues) and believe this is not a duplicate
* [x] This is not something which has obviously "already been decided" in previous versions of F#. If you're questioning a fundamental design decision that has obviously already been taken (e.g. "Make F# untyped") then please don't submit it.

Please tick all that apply:
* [x] This is not a breaking change to the F# language design
* [ ] I or my company would be willing to help implement and/or test this

## For Readers

If you would like to see this issue implemented, please click the :+1: emoji on this issue. These counts are used to generally order the suggestions by engagement.

Start by reviewing the proposal for C#-style #define and #undef and the linked conditional-compilation reference. No source files, tests, or entry points are named, so first identify where F# compiler preprocessing and language design are handled. Done means an agreed design and implementation scope, with tests covering the new directives.

Written by the indexing model from the issue text.