As part of trying to migrate to smol-toml from @iarna/toml in Cloudflare's Wrangler, we've been running into some differences in how smol-toml renders nested tables when stringifying. For instance, the following object:

{
  a: {
    b: [
      { b: 1, c: 2 },
    ],
  },
}

is rendered by smol-toml as

[a]
[[a.b]]
b = 1
c = 2

and by @iarna/toml as

[[a.b]]
b = 1
c = 2

Both are valid, but the @iarna/toml output seems clearer and easier to work with. As such, this PR modifies the smol-toml stringify logic to not render a table key if:

1. The table has children
2. All children are themselves arrays or tables

• I've added tests
• I've run the TOML test suite, which passed