Fix Const IDL value compilation errors in codegen
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I found that some const WebIDL values fail at rust compile time.
Example:
const long long TIMEOUT_IGNORED = -1;
```
906 | ConstantSpec { name: b"TIMEOUT_IGNORED\0", value: ConstantVal::DoubleVal(-1) }]
| ^^ expected f64, found integral variable
|
= note: expected type `f64`
found type `{integer}`
```
I added a explicit cast to to fix the problem in conversions that may fail to compile.
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