interpreter: Add a flatten() method to arrays

This allows users to do two things, flatten potentially nested arrays
themselves, and, to safely convert types that may be an array to not an
array.

```meson
x = [meson.get_external_property('may_be_array)].flatten()
```
```meson
x = ['a', ['b', 'c']]
assert(x.flatten() == ['a', 'b', 'c'])
```

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diff --git a/docs/markdown/snippets/array-flatten.md b/docs/markdown/snippets/array-flatten.md
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+## Array `.flatten()` method
+
+Arrays now have a `.flatten()` method, which turns nested arrays into a single
+flat array. This provides the same effect that Meson often does to arrays
+internally, such as when passed to most function arguments.