Fixing logic typos

[RblSb]

Scanned codebase for simple typos:

• m != m dead check (maybe intended to check something else, double check)
• incorrect sl = [] || (List.mem name sl && is_new_item acc name) condition
• Show ExprOf<String> as String instead of Expr in display for return types, same as for arguments implemented above: let a = if t1 == t_dynamic then a1' else a1
• String.sub file dot_pos (String.length name - dot_pos) didn't remove "." from second extension, so Foo doesn't showing up in completion with Foo.test.hx right now