fix(cssVars): cssVarName should keep backslashe in ssr dev mode

[edison1105]

close #7823

[sodatea]

/ecosystem-ci run

[vue-bot]

📝 Ran ecosystem CI: Open

suite	result	latest scheduled
language-tools	✅ success	✅ success
nuxt	✅ success	✅ success
pinia	✅ success	✅ success
quasar	❌ failure	❌ failure
router	✅ success	✅ success
test-utils	✅ success	✅ success
vant	✅ success	✅ success
vite-plugin-vue	✅ success	✅ success
vitepress	✅ success	✅ success
vue-i18n	✅ success	✅ success
vue-macros	❌ failure	❌ failure
vuetify	✅ success	✅ success
vueuse	✅ success	✅ success
vue-simple-compiler	✅ success	✅ success

[github-actions]

Size Report

Bundles

File	Size	Gzip	Brotli
runtime-dom.global.prod.js	86.7 kB	33 kB (-21 B)	29.7 kB
vue.global.prod.js	133 kB	49.8 kB (+32 B)	44.7 kB

Usages

Name	Size	Gzip	Brotli
createApp	48.2 kB	19 kB (-27 B)	17.4 kB
createSSRApp	51.5 kB	20.3 kB (-16 B)	18.5 kB
defineCustomElement	50.6 kB	19.7 kB (-35 B)	18 kB
overall	61.6 kB	23.8 kB (-16 B)	21.7 kB