chore(core): refine locale config

[NozomuIkuta]

Related to #14, #27

• Define as much config as possible at locale level
  • With this PR we can set up OG/Twitter config for each locale
• Provide defineLocaleConfig function for better DX
  • Some OG/Twitter are automatically set so that we don't have to copy and paste string literals (if set, error is thrown and we can fail fast during development)
• Add titleTemplate to index.md and ja/index.md to override titleTemplate in config.ts, because interpolation of :title is not necessary

Once CI is passed, let me merge this PR immediately.

[NozomuIkuta]

Hmm, labels are not set automatically.
I guess this is due to actions/labeler#423.

The author says it will be fixed in the new version which is currently beta.
So, at the moment, if we edit files whose mapping of label and paths conflicts with others, we have to add labels by ourselves.

I created a follow-up issue: #40