Implement "config set" subcommand

[dbarnett]

Uses toml_edit to support simple config edits like:
jj config set --repo user.email "[email protected]"

Fixes #531.

Checklist

If applicable:

• I have updated CHANGELOG.md
• I have updated the documentation (README.md, docs/, demos/)
• I have updated the config schema (src/config-schema.json)
• I have added tests to cover my changes

[dbarnett]

Hmm, my "cargo fmt" isn't wrapping these lines but the "Check formatting" insists the long line needs to be wrapped. Any idea why? I just did rustup update.

-    // TODO(#531): Infer types based on schema, and support --type arg to explicitly specify.
+    // TODO(#531): Infer types based on schema, and support --type arg to explicitly
+    // specify.

[martinvonz]

Hmm, my "cargo fmt" isn't wrapping these lines but the "Check formatting" insists the long line needs to be wrapped. Any idea why? I just did rustup update.

-    // TODO(#531): Infer types based on schema, and support --type arg to explicitly specify.
+    // TODO(#531): Infer types based on schema, and support --type arg to explicitly
+    // specify.

We use the nightly version in CI (I don't remember if there's a good reason for that), so try cargo +nightly fmt and see if that's the difference.

[yuja]

LGTM, thanks.