Add new z-index token names (Shopify#8245)

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### WHY are these changes introduced?

We want to rename these tokens in v11. By adding the new names in v10 we
can break up the major version bump into more manageable chunks (we can
create and run migrations for the values in v10 and then just remove the
values in the launch of v11).

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  Context about the problem that’s being addressed.
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### WHAT is this pull request doing?

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Adds a set of identical z-index tokens with the names `--p-z-index-*`
(vs. `--p-z-*`)

---------

Co-authored-by: Alex Page <hi@alexpage.dev>
Co-authored-by: Aaron Casanova <32409546+aaronccasanova@users.noreply.github.com>

Author: 3 people

.changeset/fuzzy-timers-kneel.md

@@ -0,0 +1,5 @@
+---
+'@shopify/polaris-tokens': minor
+---
+
+Added new z-index tokens that will be the default in v7.0.0

polaris-tokens/src/token-groups/zIndex.ts

@@ -1,6 +1,42 @@
 import type {TokenGroup} from '../types';
 
 export const zIndex = {
+  'z-index-1': {
+    value: '100',
+  },
+  'z-index-2': {
+    value: '400',
+  },
+  'z-index-3': {
+    value: '510',
+  },
+  'z-index-4': {
+    value: '512',
+  },
+  'z-index-5': {
+    value: '513',
+  },
+  'z-index-6': {
+    value: '514',
+  },
+  'z-index-7': {
+    value: '515',
+  },
+  'z-index-8': {
+    value: '516',
+  },
+  'z-index-9': {
+    value: '517',
+  },
+  'z-index-10': {
+    value: '518',
+  },
+  'z-index-11': {
+    value: '519',
+  },
+  'z-index-12': {
+    value: '520',
+  },
   'z-1': {
     value: '100',
   },