Adds an optional `fix.tools.TOOL.enabled` config that disables use of a fix
tool (if omitted, the tool is enabled). This is useful for defining tools in
the user's configuration without enabling them for all repositories:
```toml
# ~/.jjconfig.toml
[fix.tools.rustfmt]
enabled = false
command = ["rustfmt", "--emit", "stdout"]
patterns = ["glob:'**/*.rs'"]
```
Then to enable it in a repository:
```shell
$ jj config set --repo fix.tools.rustfmt.enabled true
```
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