Fix indent and is_last variable misusage

[Meredith2328]

The original src/ast/tree.rs prints extra vertical bar lines, making the AST tree less readable.

• Remove incorrectly added ! on is_last to fix extra vertical bar lines printed for Program/FnDef and other nodes
• Change VarDeclStmt::fmt_tree to pass new_indent instead of indent_levels to its inner, fixing the issue where child nodes appear at the same level as their parent

Pic1: Before fix - Program/FnDef shows extra vertical bars; VarDeclStmt and a: int are at the same level

Pic2: After fix