Add GetSubTextFromRange to ISourceText

[nojaf]

In Fantomas, we have an extension for ISourceText to grab the text based on a range.

I find myself copying this over to other projects that consume FCS quite often, and I would like to have this in FCS.

[psfinaki]

I'd say this can go in - but just trying to understand, can this be redesigned to actually not have duplicate implementations?

[nojaf]

I guess, any suggestions on where to extract the code?

[psfinaki]

I guess at least this part

else
    (let sourceText = this :> ISourceText)
    let startLine = range.StartLine - 1
    let line = sourceText.GetLineString startLine
    ...

can be moved to a method in SourceText and then reused elsewhere.

[nojaf]

The module SourceText is defined underneath the type.

fsharp/src/Compiler/Facilities/prim-lexing.fs

Line 147 in 11b1934

module SourceText =