Update lexical-analysis.md keywords for current F# syntax

[roboz0r]

Updated based on the Microsoft keyword reference and manual testing.

It's noteworthy that the Microsoft reference lists sig and mod as OCaml compatibility reserved keywords but in testing, the compiler reports sig as a full keyword and mod can be used without warnings or errors as an infix operator:

let (mod) x y = x + y
let z = 1 mod 1

[roboz0r]

After doing a bit more reading I found that some of my edits are actually included in Chapter 19

This section primarily describes additional tokens emitted and I wonder if that content would be better moved to be a subheading within lexical analysis?

[Martin521]

Thanks a lot for contributing to fslang-spec.
It seems this PR is adding FS-1016 to the spec. Can you adapt the PR title accordingly? And please update the RFC table.

In that RFC, "fixed" is not mentioned.
Can you check if it was removed by another RFC? Or when it was removed?

As for chapter 19, we would prefer to keep the chapter structure for now as is. Can you instead add a remark in chapter 3, linking to 19?

[Martin521]

Some more insights.

• Chapter 19 will be updated when FS-1114 is added.
• sig is listed in the spec as keyword, the MS F# language guide should be updated
• mod is indeed is a keyword that can be used as operator. I think it should be added to the keyword list in chapter 3, and also the special use should be mentioned. See also this comment.
• fixed was made a proper keyword in FS-1015
• const it seems was added with type providers but at the time not added to the spec keyword list, so adding it now seems to be correct.

[roboz0r]

I've split up this PR to #63 #64 #65 #66 #67 by their separate features / RFC