Reads a list of variables declarations

```ebnf
variable_declaration ::= T_VARIABLE ('=' expr)?*
variable_declarations ::= variable_declaration (',' variable_declaration)*
```

Sample code :
```php
<?php static $a = 'hello', $b = 'world';
```
@return {StaticVariable[]} Returns an array composed by a list of variables, or
assign values
function() {
    return this.read_list(function() {
      const node = this.node("staticvariable");
      let variable = this.node("variable");
      // plain variable name
      if (this.expect(this.tok.T_VARIABLE)) {
        const name = this.text().substring(1);
        this.next();
        variable = variable(name, false, false);
      } else {
        variable = variable("#ERR", false, false);
      }
      if (this.token === "=") {
        return node(variable, this.next().read_expr());
      } else {
        return variable;
      }
    }, ",");
  }