Add check_library_structure function

[per1234]

• Valid library folder name.
  • According to the Arduino IDE: "Library names must contain only basic letters and numbers. (ASCII only and no spaces, and it cannot start with a number)"
    • This is referring to the folder name, not the "fancy name" from library.properties.
    • Starting with a number is not allowed with Arduino IDE 1.8.3 and older (OK with 1.8.4 and newer).
    • I've verified that no spaces are allowed.
  • There is also a limitation of 63 characters max.
    • Exceeding this limit blocks the "Add .ZIP Library" installation method.
    • If manually installed, there are also warnings displayed on IDE startup, Library/Boards Manager open/close.
• Header file or library.properties in library root folder (the library path is passed to the function).
  • Recognized header file extensions: .h, .hh. .hpp
    • Only that specific extension case is recognized, even on Windows.
• If no header files in root folder, then headers under src folder.
  • (verified) Incorrect case of src folder name causes 1.5 format libraries to not be recognized on filename case-sensitive OS like Linux.
• If in 1.5 format, then no utility folder in root.
• All sketch files under either examples or extras folder.
  • Recognized sketch extensions are .ino, .pde
    • Only that specific extension case is recognized on case-sensitive OS like Linux.
  • Just ignore the extras folder, as the specification states it should be.
• Run check_sketch_structure on all sketch files.