fix: convert string coverage reporters to array

[AriPerkkio]

Fixes issue where coverage: { reporters: 'text' } crashes coverage report generation. String reporter is converted into array of characters and looped here:

vitest/packages/coverage-istanbul/src/provider.ts

Line 112 in 14279d5

for (const reporter of this.options.reporter) {

C8 is already converting string reporters into an array before looping them. Applied the same logic to istanbul.

vitest/packages/coverage-c8/src/provider.ts

Line 121 in 14279d5

resolved.reporter = Array.isArray(resolved.reporter) ? resolved.reporter : [resolved.reporter]

Error seen with string reporters:

export default defineConfig({
    test: {
        coverage: {
            provider: 'istanbul',
            enabled: true,
            reporter: 'text'
        }
    },
})

⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯ Unhandled Error ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯
 ❯ Function.Module._resolveFilename node:internal/modules/cjs/loader:933:15
Error: Cannot find module 't'
Require stack:
- /workspaces/vitest/node_modules/.pnpm/istanbul-reports@3.1.5/node_modules/istanbul-reports/index.js
 ❯ Function.Module._load node:internal/modules/cjs/loader:778:27
 ❯ Module.require node:internal/modules/cjs/loader:1005:19
 ❯ require node:internal/modules/cjs/helpers:102:18
 ❯ Object.create ../vitest/node_modules/.pnpm/istanbul-reports@3.1.5/node_modules/istanbul-reports/index.js:19:20
 ❯ IstanbulCoverageProvider.reportCoverage ../vitest/packages/coverage-istanbul/dist/provider-c0b24cef.js:253:15
 ❯ Vitest.start ../vitest/packages/vitest/dist/chunk-vite-node-externalize.1a5aabf5.mjs:9553:7
 ❯ startVitest ../vitest/packages/vitest/dist/chunk-vite-node-externalize.1a5aabf5.mjs:10322:5
 ❯ start ../vitest/packages/vitest/dist/cli.mjs:668:9
 ❯ CAC.run ../vitest/packages/vitest/dist/cli.mjs:664:3

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Serialized Error: {
  "code": "MODULE_NOT_FOUND",
  "requireStack": [
    "/workspaces/vitest/node_modules/.pnpm/istanbul-reports@3.1.5/node_modules/istanbul-reports/index.js",
  ],
}