chore(deps): remove shellexpand dependency

[gakonst]

Summary

Remove shellexpand dependency which is broken with the latest nightly due to str::as_str() stabilization (rust-lang/rust#130366) shadowing Cow::as_str().

Changes

• Add reusable expand_tilde() and expand_path() helpers in reth_node_core::utils
• Replace shellexpand::full() in reth-node-core with expand_path()
• Replace shellexpand::tilde() in reth-bench-compare with reth_node_core::utils::expand_tilde()
• Replace shellexpand::full() in reth-cli with local expand_tilde() (using dirs-next)
• Remove shellexpand from workspace deps

Testing

cargo +nightly clippy --workspace --lib --examples --tests --benches --all-features --locked passes clean.

Prompted by: mattsse

[github-actions]

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Preview

---
reth-bench-compare: patch
reth-cli: patch
reth-node-core: patch
---

Removed the `shellexpand` dependency and replaced it with a custom `expand_tilde` and `expand_path` implementation using `dirs-next`. Updated `parse_path` and related `FromStr` error types to use `VarError` directly instead of `shellexpand::LookupError<VarError>`.

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[GrigorenkoPV]

The stabilization of str::as_str is currently being reverted in rust-lang/rust#152963

[mattsse]

can wait for fix and pin ci until then