fix: nimcheck improvements for generics, JS operators, Python 3.10+ syntax

- Add tolerantMode for generic Nim code with < > brackets
- Fix JS ?. and ?? operator tokenization
- Add Python 3.10 match/case keywords, underscore prefix support
- Fix f-string parsing operator precedence bug
- Fix duplicate foundClosing assignments in JS/PHP tokenizers
- Treat unknown flavors as info instead of error
- Add comprehensive test coverage for all fixes

tests/fixtures/nim/with_generics.nim

@@ -0,0 +1,14 @@
+## Nim code with generics - should validate cleanly (no false "Unclosed '<'")
+proc jarray[T](node: T): T =
+  result = node
+
+proc pair[A, B](a: A, b: B): (A, B) =
+  (a, b)
+
+proc findIf[T](items: seq[T], pred: proc(x: T): bool): Option[T] =
+  for item in items:
+    if pred(item):
+      return some(item)
+
+proc compare[T](a, b: T): bool =
+  a < b and b > a