Support new slice syntax

[nojaf]

Issue created from fantomas-ui

Support https://github.com/fsharp/fslang-design/blob/master/RFCs/FS-1076-from-the-end-slicing.md

Code

let a = list.[..^0]   // 1,2,3,4,5
let b = list.[..^1]   // 1,2,3,4
let c = list.[0..^1]  // 1,2,3,4
let d = list.[^1..]   // 4,5
let e = list.[^0..]   // 5
let f = list.[^2..^1] // 3,4

Result

let a = list.[..0] // 1,2,3,4,5
let b = list.[..1] // 1,2,3,4
let c = list.[0..1] // 1,2,3,4
let d = list.[1..] // 4,5
let e = list.[0..] // 5
let f = list.[2..1] // 3,4

Options

Fantomas Next - 3.2.0-2/10/2020

Name	Value
IndentOnTryWith	false
IndentSpaceNum	4
KeepNewlineAfter	false
MaxIfThenElseShortWidth	40
PageWidth	120
ReorderOpenDeclaration	false
SemicolonAtEndOfLine	false
SpaceAfterComma	true
SpaceAfterSemicolon	true
SpaceAroundDelimiter	true
SpaceBeforeArgument	true
SpaceBeforeColon	false
StrictMode	false