feat: add user-defined color palette (#4209)

* docs(config): add color palette to docs

* feat: add user-defined color palette

* fix: update config schema

* refactor: apply suggestions from code review

Co-authored-by: David Knaack <davidkna@users.noreply.github.com>

* fix: update new test

* feat: add support for multiple palettes

* docs(config): update docs for multiple color palettes

* docs(config): fix formatting

* test: test overriding a predefined color with itself

* docs: mention palettes cannot reference themselves

* refactor: warn when using a nonexistent palette

* test: test retrieving a nonexistent color palette

* fix: fix issues with palette log messages

* fix: update config schema

* fix: skip serializing palette if none

* refactor: change nonexistent palette message to warning

* fix: update config schema

Co-authored-by: David Knaack <davidkna@users.noreply.github.com>

.github/config-schema.json

@@ -1544,6 +1544,22 @@
     "add_newline": {
       "default": true,
       "type": "boolean"
+    },
+    "palette": {
+      "type": [
+        "string",
+        "null"
+      ]
+    },
+    "palettes": {
+      "default": {},
+      "type": "object",
+      "additionalProperties": {
+        "type": "object",
+        "additionalProperties": {
+          "type": "string"
+        }
+      }
     }
   },
   "additionalProperties": false,