Add conditionals

[BLM16]

• if statements
• else if statements
• else statements

Boolean expressions that determine whether or not to run the code should not require parentheses, however they should be optional. expression contains a nested '(' expression ')' already.

if_statement: 'if' expression code_block

Potentially add an inline syntax similar to

if_statement: 'if' expression (code_block | 'then' statement)

---

if number == 37 {
    do_something();
}

if number == 37 then do_something();