Fix mode generation for plugin-sass + plugin-css (#202)

.changeset/fast-wolves-shake.md

@@ -0,0 +1,5 @@
+---
+"@cobalt-ui/cli": patch
+---
+
+Throw error on missing tokens.json

.changeset/hip-llamas-sip.md

@@ -0,0 +1,7 @@
+---
+'@cobalt-ui/plugin-sass': minor
+---
+
+Fix collision of mode names if a user has a mode called `"default"`, changing the internal selector to `"."` (a single dot).
+
+`"."` will be used more often in the upcoming [2.0 API](https://github.com/drwpow/cobalt-ui/issues/201), chosen because it’s easy to type/reference, uses illegal characters (therefore consumers can’t use it, but internally we know what it means), and also because it matches [Node’s ESM implementation for the default import](https://nodejs.org/docs/latest/api/esm.html#resolution-algorithm-specification), so it will be familiar/expected for some developers.

.changeset/shiny-rats-grow.md

@@ -0,0 +1,5 @@
+---
+"@cobalt-ui/plugin-sass": patch
+---
+
+Fix plugin-sass mode generation