Refactor ExpressionRunner (#2804)

Tackles the concerns raised in https://github.com/WebAssembly/binaryen/issues/2797 directly related to https://github.com/WebAssembly/binaryen/pull/2702 by reverting merging all of `PrecomputeExpressionRunner` into the base `ExpressionRunner`, instead adding a common base for both the precompute pass and the new C-API to inherit. No functional changes.

---

### Current hierarchy after https://github.com/WebAssembly/binaryen/pull/2702 is

```
ExpressionRunner
├ [PrecomputeExpressionRunner]
├ [CExpressionRunner]
├ ConstantExpressionRunner
└ RuntimeExpressionRunner
```

where `ExpressionRunner` contains functionality not utilized by `ConstantExpressionRunner` and `RuntimeExpressionRunner`.

### New hierarchy will be:

```
ExpressionRunner
├ ConstantExpressionRunner
│  ├ [PrecomputeExpressionRunner]
│  └ [CExpressionRunner]
├ InitializerExpressionRunner
└ RuntimeExpressionRunner
```

with the precompute pass's and the C-API's shared functionality now moved out of `ExpressionRunner` into a new `ConstantExpressionRunner`. Also renames the previous `ConstantExpressionRunner` to `InitializerExpressionRunner` to [better represent its uses](https://webassembly.org/docs/modules/#initializer-expression) and to make its previous name usable for the new intermediate template, where it fits perfectly. Also adds a few comments answering some of the questions that came up recently.

### Old hierarchy before https://github.com/WebAssembly/binaryen/pull/2702 for comparison:

```
ExpressionRunner
├ [PrecomputeExpressionRunner]
├ ConstantExpressionRunner
└ RuntimeExpressionRunner
```

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