ironhtml-parser: std feature flag does nothing

[dannywillems]

Description

`crates/ironhtml-parser/Cargo.toml:19-21` declares an `std` feature:

```toml
[features]
default = []
std = []
```

The feature gates `#![cfg_attr(not(feature = "std"), no_std)]` but there are no `#[cfg(feature = "std")]` conditional code blocks in the parser. The feature exists but enables nothing beyond removing the `no_std` attribute.

Should either be removed or used to provide std-specific functionality (e.g., `impl std::error::Error`).