Support MsBuild syntax for setting warnings easily

[baronfel]

Right now there's a lot of overhead to adding --warnon values to a project's compilation. The current syntax requires a user to do something like

<OtherFlags>$(OtherFlags) --warnon:`warningNumber`</OtherFlags>

for each warning they want to set, especially if they have multiple warnings and want do document what each is added for.

This is error prone, because a user has to:

• remember to insert any previous flags (or else they will be overwritten
• remember --warnon syntax

Describe the solution you'd like
I'd like for syntax like <WarnOn>warningNumber</WarnOn> to be supported in the F# MsBuild targets and Fsc task.

This syntax would allow for multiple-definitions of WarnOn and roll each definition up into a final list, which would make it easy to add and comment specific warnings.
It would also be easy to add warnings to the compiler at the targets level instead of baking them into the compiler, if that was desired (see the new 33390 xml validation errors/warnings) as an example.

Describe alternatives you've considered
Keep the status quo.

Additional context
Initially brought up here, but I've been wanting this for ages