[Feature request] Add cast function with sugared syntax

# Problem
MethodScript currently does not have a method to cast to any desired type. This makes it impossible to fully statically type programs such as:
```java
mixed @a = ...;
if(@a instanceof array) {
array @arr = @a; // Typecheck error, need to insert a cast.
}
```

# Proposed solution
- Add a `cast(mixed, classtype)` function that casts the given mixed to the given classtype, or throws a `ClassCastException` when it can't.
- Add `(type) expression` syntax that compiles to `cast(expression, type)`.

Note that MethodScript currently does have `integer(mixed)`, `string(mixed)`, `boolean(mixed)` and `double(mixed)` that all cast to one specific type.

该 issue 描述了一个新的 cast(mixed, classtype) 函数和带语法糖的 (type) 表达式语法，包括 ClassCastException 的行为以及一个类型检查示例。由于没有指定源文件、测试或入口点，请先定位现有的 integer、string、boolean 和 double 转换，以及表达式的 parser 和类型检查路径。完成的标准是两种形式都能转换为请求的类型，并按指定拒绝不兼容的值。

由索引模型根据 Issue 内容生成。