Move mustache typings to devDependencies (elastic#24165)

This moves the `@types/mustache` package from `dependencies` to `devDependencies` to avoid unnecessary inclusion in the build artifacts.

package.json

@@ -76,7 +76,6 @@
     "@kbn/pm": "link:packages/kbn-pm",
     "@kbn/test-subj-selector": "link:packages/kbn-test-subj-selector",
     "@kbn/ui-framework": "link:packages/kbn-ui-framework",
-    "@types/mustache": "^0.8.31",
     "JSONStream": "1.1.1",
     "abortcontroller-polyfill": "^1.1.9",
     "angular": "1.6.9",
@@ -243,6 +242,7 @@
     "@types/listr": "^0.13.0",
     "@types/lodash": "^3.10.1",
     "@types/minimatch": "^2.0.29",
+    "@types/mustache": "^0.8.31",
     "@types/node": "^8.10.20",
     "@types/prop-types": "^15.5.3",
     "@types/puppeteer": "^1.6.2",