Add GDScript export hints for named bit flags, exponential ranges and global filesystem

Created by: leonkrause

See discussion in #2988 (closed) This change exposes remaining sensible export hints:

• Named bit flags (PROPERTY_HINT_FLAGS)

  export( int, FLAGS, "Fire", "Water", "Earth", "Wind" )

• Exponential ranges (PROPERTY_HINT_EXP_RANGE)

  export( float, EXP, 100, 200, 2 )

• Global filesystem directories (PROPERTY_HINT_GLOBAL_DIR) restricted to tool scripts

  export( String, DIR, GLOBAL )

• Global filesystem files (PROPERTY_HINT_GLOBAL_FILE) restricted to tool scripts

  export( String, FILE, GLOBAL, ".txt" )

Closes #2988 (closed)