Add starOfStream function

src_v2/parser/parser_api.nim

@@ -112,6 +112,21 @@ proc str*(expectedString: string): parserFnT {.inline.} =
       res = res.flatMap(ch(c))
     return res
 
+proc startOfStream*(parser: Parser): ParserResult =
+  ## Check if the following character is the end of the stream.
+  ## Errors when the end of stream was already reached.
+  let position = parser.state.position
+
+  if position == 0 or position == -1:
+    ok(parser)
+  else:
+    err(ParserError(
+      kind: startOfStringErr,
+      expected: "startOfStream",
+      index: position,
+      parser: parser,
+    ))
+
 proc endOfStream*(parser: Parser): ParserResult =
   ## Check if the following character is the end of the stream.
   ## Errors when the end of stream was already reached.