Rename --line-height-* variables to --leading-*

[philipp-spiess]

Part of the current changes, we also want to make the --line-height-* namespace closer to the utility name so we're renaming it to --leading-*:

  @theme {
-  --line-height-none: 1;
-  --line-height-tight: 1.25;
-  --line-height-snug: 1.375;
-  --line-height-normal: 1.5;
-  --line-height-relaxed: 1.625;
-  --line-height-loose: 2;

    /* ... */
  
+  --leading-none: 1;
+  --leading-tight: 1.25;
+  --leading-snug: 1.375;
+  --leading-normal: 1.5;
+  --leading-relaxed: 1.625;
+  --leading-loose: 2;

    /* ... */
  }

Notice that we did not change the nested values used in the --text type scale, e.g.:

@theme {
  /* Type scale */
  --text-xs: 0.75rem;
  --text-xs--line-height: 1rem;
}

These do not refer to the leading utility and instead refer to nested properties so we're keeping those as-is.

Test Plan

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