Add LintFile method to API layer

[MrLuje]

Branched from #637, so it should be merged first

See MrLuje/FSharpLint@9635b9a...MrLuje:FSharpLint:lint_layer diff for relevant changes

WIP on #627:

• Expose a lint method (that will call val lintFile : optionalParams:OptionalLintParameters -> filePath:string -> LintResult method)

For reminder, the purpose of the API layer is to abstract away all FCS types so the caller can have a different FCS version than FSharpLint.

I duplicated types from https://github.com/fsprojects/FSharpLint/blob/master/src/FSharpLint.Core/Framework/Suggestion.fs to FSharpLint.Client and prefixed them with Client (eg: SuggestedFix => ClientSuggestedFix) to tell them apart. (I created ClientRange from scratch to avoid the FCS dependency) but that's open to discussion.

type LintFileRequest =
    {
        FilePath: string
        LintConfigPath: string option
    }

For now, the LintFileRequest takes the path to the file to lint and an optional LintConfigPath to a fsharplint.json file but I'm wondering about other scenario :

• If no config is specified, defaultConfiguration will be used (same with an invalid LintConfigPath)
• If a valid fsharplint.json is specified through LintConfigPath, it will be used
• What about if LintConfigPath isn't specified but there is an existing fsharplint.json in the project ? is there a standard location for fsharplint.json or an existing detection method ? (next to solution file, parse [fc]sproj, etc)