replace _ in the CSS hash

Just happened to run into this in the release branch...
```diff
- --- ./dist/assets/index-<hash>.css ---
+ --- ./dist/assets/index-B_aD48aL.css ---
  .do-include-me {
    color: green;
  }
```

For context, when we deal with Vite related integration tests, we build
a CSS file, and dump all the generated files (file path + contents).
Since Vite uses a hash in the filename, we stripped it and replaced it
with `<hash>` because the hash doesn't matter for the tests.

We just happen to run into a situation where a `_` character was used
and our regex didn't match.

Long story short, this is a small internal change that doesn't need any
kind of changelog entry.

Author: RobinMalfait

integrations/vite/resolvers.test.ts

@@ -571,8 +571,9 @@ test(
   async ({ exec, fs, expect }) => {
     await exec('pnpm vite build')
 
-    expect((await fs.dumpFiles('./dist/**/*.css')).replace(/-([a-zA-Z0-9]*?)\.css/g, '-<hash>.css'))
-      .toMatchInlineSnapshot(`
+    expect(
+      (await fs.dumpFiles('./dist/**/*.css')).replace(/-([_a-zA-Z0-9]*?)\.css/g, '-<hash>.css'),
+    ).toMatchInlineSnapshot(`
       "
       --- ./dist/assets/index-<hash>.css ---
       .do-include-me {
@@ -641,8 +642,9 @@ test(
   async ({ exec, fs, expect }) => {
     await exec('pnpm vite build')
 
-    expect((await fs.dumpFiles('./dist/**/*.css')).replace(/-([a-zA-Z0-9]*?)\.css/g, '-<hash>.css'))
-      .toMatchInlineSnapshot(`
+    expect(
+      (await fs.dumpFiles('./dist/**/*.css')).replace(/-([_a-zA-Z0-9]*?)\.css/g, '-<hash>.css'),
+    ).toMatchInlineSnapshot(`
         "
         --- ./dist/assets/index-<hash>.css ---
         @layer base {