Add configuration file to collect test coverage

[c-bata]

Motivation

Currently, the test coverage for Python source files are collected by using pytest-cov and is sent to codecov.io. However, the test coverage for our TypeScript files remains unaccounted.

Suggestion

Please refer to the official documentation of Vitest, https://vitest.dev/guide/coverage, and add a configuration file to collect the test coverage for @optuna/react package, which is located on tslib/react.

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