Allow unsetting config values similar to `git config unset`.
```bash
$ jj config set --user some-key some-val
$ jj config get some-key
some-val
$ jj config unset --user some-key
$ jj config get some-key
Config error: configuration property "some-key" not found
For help, see https://martinvonz.github.io/jj/latest/config/.
```
Unsetting a key, which is part of a table, might leave that table empty.
For now we do not delete such empty tables, as there may be cases where
an empty table is semantically meaningful
(https://github.com/martinvonz/jj/issues/4458#issuecomment-2407109269).
For example:
```toml
[table]
key = "value"
[another-table]
key = "value"
```
Running `jj config unset --user table.key` will leave us with `table`
being empty:
```toml
[table]
[another-table]
key = "value"
```
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