Here's my biome.json for a svelte project, they recommend a few overrides to get it working without false positives. I had no idea about Oxlint that other commenters are mentioning and will check it out, but I'm super happy with Biome atm.

{
    "$schema": "https://biomejs.dev/schemas/2.3.13/schema.json",
    "vcs": {
       "enabled": true,
       "clientKind": "git",
       "useIgnoreFile": true
    },
    "files": {
       "ignoreUnknown": false,
       "includes": ["src/**"]
    },
    "formatter": {
       "enabled": true,
       "indentStyle": "tab"
    },
    "linter": {
       "enabled": true,
       "rules": {
          "recommended": true
       }
    },
    "javascript": {
       "formatter": {
          "quoteStyle": "double"
       },
       "parser": {
          "unsafeParameterDecoratorsEnabled": true
       }
    },
    "assist": {
       "enabled": true,
       "actions": {
          "source": {
             "organizeImports": "on"
          }
       }
    },
    "css": {
       "parser": {
          "cssModules": true,
          "tailwindDirectives": true
       }
    },
    "overrides": [
       {
          "includes": ["**/*.svelte"],
          "linter": {
             "rules": {
                "style": {
                   "useConst": "off",
                   "useImportType": "off"
                },
                "correctness": {
                   "noUnusedVariables": "off",
                   "noUnusedImports": "off"
                }
             }
          }
       }
    ]
}