Rename --font-size-* variables to --text-*

[adamwathan]

This PR updates all of the --font-size-* variables to --text-* instead to closer match the utility names.

  @theme {
-   --font-size-xs: 0.75rem;
-   --font-size-xs--line-height: 1rem;
-   --font-size-sm: 0.875rem;
-   --font-size-sm--line-height: 1.25rem;
-   --font-size-base: 1rem;
-   --font-size-base--line-height: 1.5rem;
-   --font-size-lg: 1.125rem;
-   --font-size-lg--line-height: 1.75rem;
-   --font-size-xl: 1.25rem;
-   --font-size-xl--line-height: 1.75rem;

    /* ... */
  
+   --text-xs: 0.75rem;
+   --text-xs--line-height: 1rem;
+   --text-sm: 0.875rem;
+   --text-sm--line-height: 1.25rem;
+   --text-base: 1rem;
+   --text-base--line-height: 1.5rem;
+   --text-lg: 1.125rem;
+   --text-lg--line-height: 1.75rem;
+   --text-xl: 1.25rem;
+   --text-xl--line-height: 1.75rem;

    /* ... */
  }

This is part of a bigger set of changes where we're renaming other theme variables as well with the same goals, since many existing theme variables like --shadow-* and --radius-* are already not using the explicit CSS property name.