Support for sprites on serve

[keichan34]

Closes #137, closes #62

When writing sprite support, I saw that there were no tests for the charites serve command, so I added some simple tests. While there were initially tests in #80, they were removed before merging because of some timeout issue. I think I've resolved those issues in this PR.

Description

Adds sprite support to the serve command. Uses the same --sprite-input command line option as build.

Type of Pull Request

• [x] Adding a feature
• [ ] Fixing a bug
• [ ] Maintaining documents
• [ ] Others ()

Verify the followings

• [x] Code is up-to-date with the main branch
• [x] No build errors after npm run build
• [x] No lint errors after npm run lint
• [x] No errors on using charites help globally
• [x] Make sure all the existing features working well
• [x] Have you added at least one unit test if you are going to add new feature?
• [x] Have you updated documentation?

[keichan34]

@naogify

If user has an icons directory, I think charites should show local icons without explicitly specifying --sprite-input.

I agree. But, I don't think it's in the scope of this PR. I think that charites serve and charites build should have the same interface for building sprites -- this is a big part of why issues like #115 and #47 exist. I've opened an issue (#146) about unifying the interface and maybe specifying a special key in style.yml that indicates that sprites should be built, so sprites could be built automatically without specifying --sprite-input both on serve and build.

[naogify]

@keichan34 I understand it. I think #146 is nice idea! We have so much command line options right now. Thank you!