Configurable name for _shell.html

[phi0411141]

Currently tanstack start build (with spa enabled) produces no index.html but only _shell.html. I see this github discussion #3394, but this will likely to break 95% traditional spa production server. I recommend to make the _shell.html filename custom-able, so at least users can slowly update their tooling
Below is my current config for re-production

import { defineConfig } from 'vite'
import { tanstackStart } from '@tanstack/react-start/plugin/vite'
import tsConfigPaths from 'vite-tsconfig-paths';
import {lingui} from '@lingui/vite-plugin';
import legacy from '@vitejs/plugin-legacy';
import tailwindcss from "tailwindcss";

export default defineConfig({
    plugins: [
        tsConfigPaths({
            projects: ['./tsconfig.json'],
        }),
        lingui(),

        tanstackStart({
            spa: {
                enabled:true
            },
            react: {
                babel: {
                    plugins: ['@lingui/babel-plugin-lingui-macro'],
                },
            },
            target: 'static',

        }),
        legacy({
            targets: ['defaults', 'not IE 11'],
            renderLegacyChunks: false,
            polyfills: ['es/object/has-own'],
        }),
    ],
    css: {
        postcss: {
            plugins: [tailwindcss()],
        },
    },
})