add canonicalize utilities with 0 test

This will rely on 2 things:

1. That we properly constant fold length units into `0`
2. That we prefer the "positive" version (`mt-0`) over its negative version (`-mt-0`)

packages/@tailwindcss-upgrade/src/codemods/template/migrate.test.ts

@@ -62,8 +62,8 @@ describe.each([['default'], ['with-variant'], ['important'], ['prefix']])('%s',
     //
     // This translation is actually fine, because now, we will prefer the
     // non-negative version first so we can replace `-mt-[0px]` with `mt-[0px]`.
-    ['mt-[0px]', 'mt-[0px]'],
-    ['-mt-[0px]', 'mt-[0px]'],
+    ['mt-[0px]', 'mt-0'],
+    ['-mt-[0px]', 'mt-0'],
 
     // Shorthand CSS Variables should be converted to the new syntax, even if
     // the fallback contains functions. The fallback should also be migrated to

packages/tailwindcss/src/canonicalize-candidates.test.ts

@@ -675,6 +675,23 @@ describe.each([['default'], ['with-variant'], ['important'], ['prefix']])('%s',
       ['grid-cols-[subgrid]', 'grid-cols-subgrid'],
       ['grid-rows-[subgrid]', 'grid-rows-subgrid'],
 
+      // Handle zeroes
+      ['m-[0]', 'm-0'],
+      ['m-[0px]', 'm-0'],
+      ['m-[0rem]', 'm-0'],
+
+      ['-m-[0]', 'm-0'],
+      ['-m-[0px]', 'm-0'],
+      ['-m-[0rem]', 'm-0'],
+
+      ['m-[-0]', 'm-0'],
+      ['m-[-0px]', 'm-0'],
+      ['m-[-0rem]', 'm-0'],
+
+      ['-m-[-0]', 'm-0'],
+      ['-m-[-0px]', 'm-0'],
+      ['-m-[-0rem]', 'm-0'],
+
       // Only 50-200% (inclusive) are valid:
       // https://developer.mozilla.org/en-US/docs/Web/CSS/font-stretch#percentage
       ['font-stretch-[50%]', 'font-stretch-50%'],