A round of deprecation of OCaml compatibility language features

[dsyme]

I propose we deprecate certain existing corner-case features of F#

• Make the use of the ".ml" and ".mli" file extensions give errors on command line and IDE by default and no longer give direction to the "/mlcompatibility" flag but rather go via an https://aka.ms link explaining this feature is being removed (they currently give textual warnings directing to use "/mlcompatibility")

• Make the “/mlcompatibility” flag hidden and give a deprecation warning on use (it is currently accepted without a warning) with the same https://aka.ms link

• This also covers the (* IF-OCAML .. *) and other things that require "/mlcompatibility".

• When "#indent “off” and #light “off” are used (in command line compiler or the IDE) they should give the same error and link - again requiring "/mlcompatibility" to explicitly enable. This will not be available at all in scripts except by starting fsi with "/mlcompatibility", there will no option to enable this without an error in script editing.

  (This maybe more controversial as I believe there have historically been people using F# who prefer the indentation-off syntax. However in reality new features since F# 4.0 have only been tested with the indentation-aware default syntax. It's time to embrace this as the reality for the language)

• The same thing applies for the infix keywords "land", "lor", "lxor", "lsr", "lsl", "mod”, "asr". These were in F# 1.0 in the language for OCaml compat and are never encouraged to be used. They should not have made it into F# 2.0+ as a "warning" and should always have been an "error".

• Give a deprecation warning for ! and instead request the use of .Value, covered in this issue Give advisory messages when using ! and :=, and ask users to use .Value instead  #569. This may be more controversial and will affect a substantial amount of F# code. See the separate issue which has already been approved in principle.

---

I feel there are a few more too though I didn’t find them on a quick scan of the codebase. There are some “error” deprecations we can trim out for code cleanliness, e.g.

   errorR(Deprecated(FSComp.SR.tcObjectExpressionFormDeprecated(), m))
   errorR(Deprecated(FSComp.SR.tcUseWhenPatternGuard(), m))
   errorR(Deprecated(FSComp.SR.tcUnnamedArgumentsDoNotFormPrefix(), mMethExpr))
   errorR(DeprecatedThreadStaticBindingWarning mBinding)
   astDeprecatedIndexerNotation
   elDeprecatedOperator

[dsyme]

I'm marking this as completed, though the implementation is still in preview. For some reason it wasn't activated in F# 6.0

https://github.com/fsharp/fslang-design/blob/main/FSharp-6.0/FS-1114-ml-compat-revisions.md