Fix #1383: Throw NotSupportedException for unrecognized CSS pseudo-classes (#1613)

Instead of the misleading 'Invalid css selector syntax' error, throw
NotSupportedException with a descriptive message when an unrecognized
CSS pseudo-class or pseudo-element (e.g. :nth-child, :first-child) is
encountered. This makes it clear the syntax is valid CSS but not yet
implemented in FSharp.Data's CSS selector engine.

Co-authored-by: Repo Assist <repo-assist@github.com>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Co-authored-by: Don Syme <dsyme@users.noreply.github.com>

Author: 4 people

src/FSharp.Data.Html.Core/HtmlCssSelectors.fs

@@ -178,6 +178,16 @@ module internal HtmlCssSelectors =
                 | StartsWith ":enabled" t -> tokenize' (Enabled(getOffset t + 1) :: acc) t
                 | StartsWith ":file" t -> tokenize' (File(getOffset t + 1) :: acc) t
                 | StartsWith ":submit" t -> tokenize' (Submit(getOffset t + 1) :: acc) t
+                | ':' :: t ->
+                    let s, _ = readString "" t
+
+                    raise (
+                        NotSupportedException(
+                            sprintf
+                                "CSS pseudo-class or pseudo-element ':%s' is not supported. See https://fsprojects.github.io/FSharp.Data/library/HtmlCssSelectors.html for the list of supported selectors."
+                                s
+                        )
+                    )
 
                 | '>' :: t ->
                     let seqtoken = acc |> List.toSeq |> Seq.skip (1) |> Seq.toList