Add gap properties to replace-spacing-lengths migration (#7342)

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### WHY are these changes introduced?

Part of #7212  <!-- link to issue if one exists -->

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### WHAT is this pull request doing?

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Adding more functionality to the `replace-spacing-lengths` migration
script.

Running ```npx @shopify/polaris-migrator replace-spacing-lengths
"**/*.scss"``` will now also target the following `gap` properties and
replace any `px`, `rem` or `rem()` values that align with our [spacing
tokens](https://github.com/Shopify/polaris/blob/a192e3641a8629a2ea9e1a1bd5d6c20aaf9a499c/polaris-tokens/src/token-groups/spacing.ts).

- `gap`
- `grid-gap`
- `row-gap`
- `grid-row-gap`
- `column-gap`
- `grid-column-gap`

Co-Authored-By: Chloe Rice <[email protected]>

Co-authored-by: Chloe Rice <[email protected]>
Co-authored-by: Chloe Rice <[email protected]>
Co-authored-by: Aaron Casanova <[email protected]>

Author: 3 people

.changeset/dirty-students-move.md

@@ -0,0 +1,6 @@
+---
+'@shopify/polaris-migrator': minor
+---
+
+Rename `replace-sass-length` migration to `replace-spacing-lengths`
+Add `gap` properties to `replace-sass-space` migration

polaris-migrator/README.md

@@ -49,6 +49,25 @@ A generic codemod to rename any component prop.
 npx @shopify/polaris-migrator rename-component-prop <path> --component=MyComponent --from=prop --to=newProp
 ```
 
+### `replace-spacing-lengths`
+
+Replace lengths and functions (`px`, `rem` and `rem()`) in spacing declarations (`padding`, `margin`, and `gap`) with the corresponding Polaris spacing token.
+
+```diff
+- padding: 16px;
++ padding: var(--p-space-4);
+
+- margin: 1rem;
++ margin: var(--p-space-4);
+
+- gap: rem(16px);
++ gap: var(--p-space-4);
+```
+
+```sh
+npx @shopify/polaris-migrator replace-spacing-lengths <path>
+```
+
 ### v9
 
 For projects that use the [`@use` rule](https://sass-lang.com/documentation/at-rules/use), all Sass related migrations (ex: `replace-sass-spacing`) accept a `namespace` flag to target a specific `<namespace>.<variable|function|mixin>`.