Currently `Config::load` merges editor configs with
`merge_toml_values(global, local, 3)`

Besides the puzzling use of the magic number `3`, this is a bug,
because it concatenates arrays from the global config file with
those from the local config file.

For example `editor.shell=["bash", "-c"]` and
`editor.shell=["fish", "-c"]` would become
`editor.shell=["bash", "-c", "fish", "-c"]`.

This commit adds a new function `merge_toml_values_with_strategy` and
changes `Config::load` to call:
```
  merge_toml_values_with_strategy(
    global,
    local,
    &MergeStrategy {
      array: MergeMode::Never,
      table: MergeMode::Always,
    },
  )
```

For backward compatability, the original function `merge_toml_values`
calls:
```
  merge_toml_values_with_strategy(
    left,
    right,
    &MergeStrategy {
      array: MergeMode::MaxDepth(max_merge_depth),
      table: MergeMode::MaxDepth(max_merge_depth),
    },
  )
```
, so there should be no change in the way themes and languages.toml
are merged.

Also update doc string and add tests.