Rename --letter-spacing-* variables to --tracking-*

[philipp-spiess]

Part of the current changes, we also want to make the --letter-spacing-* namespace closer to the utility name so we're renaming it to --tracking-*:

  @theme {
-  --letter-spacing-tighter: -0.05em;
-  --letter-spacing-tight: -0.025em;
-  --letter-spacing-normal: 0em;
-  --letter-spacing-wide: 0.025em;
-  --letter-spacing-wider: 0.05em;
-  --letter-spacing-widest: 0.1em;

    /* ... */
  
+  --tracking-tighter: -0.05em;
+  --tracking-tight: -0.025em;
+  --tracking-normal: 0em;
+  --tracking-wide: 0.025em;
+  --tracking-wider: 0.05em;
+  --tracking-widest: 0.1em;

    /* ... */
  }

Test Plan

Added cases to the CSS theme() variable/JS plugin tests (interop layer) and the integration tests (codemod layer).