Simplify crossing point algorithm and allow complex combination to joinable

Instead of having a database of combinable characters, the new algorithm uses bit-wise to detect the crossing characters.

A character is converted into 12-bit number of 3 groups of format (single, bold, double). Each group contains 4 bits representing 4 directions: [bottom, top, right, left]

 DOUBLE          SINGLE             
╠═══════╣       ├───────┤  B: Bottom
┌─┬─┬─┬─┬─┬─┬─┬─┬─┬─┬─┬─┐  T: Top   
│B│T│R│L│B│T│R│L│B│T│R│L│  R: Right 
└─┴─┴─┴─┴─┴─┴─┴─┴─┴─┴─┴─┘  L: Left  
        ┣━━━━━━━┫                   
          BOLD

2 crossing characters at a cell will be combined with or operator (inner_mask).
Equivalent 4 surrounding characters of the 2 are also combined with or operator (outer_mask).
Then, inner_mask combines with outer_mask with and operator.
The result will be decoded into characters again and that is the crossing point character.

This also allows complex combinations of single-bold and single-double chars to connect

╓───────┒
║       ┃
╟───────┨
║       ┃
╙───────┚

TODO: new complex combination characters (such as ╓) are not used as possible surrounding yet.

tuanchauict / MonoSketch

Simplify crossing point algorithm and allow complex combination to joinable #588