feat(forge): optional chain sections and skip flags in script config

### Component

Config

### Describe the feature you would like

Foundry’s current `Config` loader (used by `_loadConfig` and `_loadConfigAndForks`) attempts to parse every configured chain section in a TOML file and will error if a section is present but effectively empty or missing expected keys. This makes multi-chain workflows harder when some chains intentionally lack certain config values.

**Example config**:

```toml
[sepolia]
endpoint_url = "${SEPOLIA_RPC_URL}"

[anvil]
endpoint_url = "http://127.0.0.1:8545"
[anvil.bool]
use_mocks = true

[anvil.uint]
mock_decimals = 8
mock_initial_price = 2000e8
```

In this file:

sepolia does not define mocks — and that’s fine.

anvil does not define price_feed — also fine.

But Foundry currently will throw when trying to load or parse these mismatched sections as if all chains must have all keys present.

### Proposal
1. Skip Empty Chain Sections with Warning

Treat a chain section as optional:

If a top-level chain section has no keys, skip it with a detailed warning:

```pgsql
Warning: Skipped config section for chain `` — no values found.
```

2. Add a Skip Flag

Add a new loader option to make this behavior explicitly opt-in:

```toml
_skipEmptySections = true
```

This flag would be passed alongside _loadConfig or _loadConfigAndForks, e.g.:

```solidity
_loadConfig("./deployments.toml", true, ConfigOpts.skipEmptySections);
```

Or via a global config option at the top of the TOML:

```toml
[_config]
skip_empty_sections = true
```

3. Verbose Warnings for Missing Keys

When a chain section has some keys but is missing others seen in other chains, emit a verbose warning:

```pgsql
Warning: chain `sepolia` missing key `use_mocks`; only defined under `anvil`.
```

This helps developers see mismatches in multi-chain configs.

4. Key Lookup Behavior (Unchanged)

Only throw at runtime when a script explicitly calls config.get(...) for a missing key.

Missing keys do not cause load errors with the skip option enabled.

### Summary of Desired Behavior

| Behavior | With Skip Flag (`_skipEmptySections = true`) |
|----------|----------------------------------------------|
| Empty chain section | Skipped with a detailed warning (`no values found` for that chain) |
| Missing key under a chain | No load error; only errors if script explicitly calls `config.get(...)` |
| Cross-chain mismatches | Emit verbose warnings showing which keys are missing under which chains |
| Backwards compatibility | Default behavior unchanged; skip flag is opt-in |

This feature will significantly improve ergonomics for multi-chain workflows where not every chain has every config value defined.

### Additional context

As shown below, Patrick Collins (of Cyfrin) highlights the failure case when blank chain sections are present: