Change syntax analyzer to parse same level commands to the same level in AST

[NickSettler]

Current syntax analyzer implementation will process the following program

$a = 1;
$b = 2;

as the following tree

graph TD
  R(SYN_NODE_SEQUENCE) --L--> RL(SYN_NODE_SEQUENCE)
  R --R--> RR(SYN_NODE_ASSIGN)
  RL --L--> RLL(NULL)
  RL --R--> RLR(SYN_NODE_ASSIGN)
  RLR --L--> RLRL(SYN_NODE_INDENTIFIER)
  RLR --R--> RLRR(SYN_NODE_INTEGER)
  RR --L--> RRL(SYN_NODE_INDENTIFIER)
  RR --R--> RRR(SYN_NODE_INTEGER)

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Current implementation parses program in a kind of binary tree.

This should be changed to make syntax tree able to store multiple nodes in a particular level. So the mentioned program must be parsed into the following tree to reduce nesting of the whole tree. This implementation is more suitable unlike the second one

graph TD
  R(PROGRAM) --1--> R1(SYN_NODE_ASSIGN)
  R --2--> R2(SYN_NODE_ASSIGN)
  R --"..."--> RN(...)
  R1 --1--> R1L(SYN_NODE_INDENTIFIER)
  R1 --2--> R1R(SYN_NODE_INTEGER)
  R2 --1--> R2L(SYN_NODE_INDENTIFIER)
  R2 --2--> R2R(SYN_NODE_INTEGER)

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Also this can be parsed into the following tree. But such implementation will be hard to debug

graph TD
  R(PROGRAM) --N--> R1(SYN_NODE_ASSIGN)
  R1 --L--> R1L(SYN_NODE_INDENTIFIER)
  R1 --R--> R1R(SYN_NODE_INTEGER)
  R1 --N--> R2(SYN_NODE_ASSIGN)
  R2 --L--> R2L(SYN_NODE_INDENTIFIER)
  R2 --R--> R2R(SYN_NODE_INTEGER)
  R2 --N--> R3(...)

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