build: Set distTag to ui-v0.15 for this release branch

- Note there's still some weirdness around how we should handle dist-tags, will address with deephaven#557 fixes. This just makes sure I don't publish a version "latest" from this branch
- Update the changelogPreset to use conventionalcommits (was done in web-client-ui with deephaven/web-client-ui#2054)

lerna.json

@@ -3,5 +3,13 @@
   "useWorkspaces": true,
   "useNx": true,
   "version": "independent",
-  "packages": ["plugins/*/src/js/"]
+  "packages": ["plugins/*/src/js/"],
+  "command": {
+    "publish": {
+      "distTag": "ui-v0.15"
+    },
+    "version": {
+      "changelogPreset": "conventionalcommits"
+    }
+  }
 }

package.json

@@ -37,6 +37,7 @@
     "@types/node": "^20.11.17",
     "@types/prop-types": "^15.7.10",
     "@types/shortid": "^0.0.29",
+    "conventional-changelog-conventionalcommits": "^7.0.0",
     "eslint": "^8.37.0",
     "identity-obj-proxy": "^3.0.0",
     "jest": "^29.6.2",