Upgrade all packages to work with node 18, update legacy code for current Next version

[brendacs]

Upgraded packages

• next
• react
• react-dom
• typescript
• eslint plugins

Added packages

• unified
• remark plugins

Replaces

• next-mdx-enhanced with @next/mdx

Component updates

• add legacyBehavior prop to Link
• rename Layout's propdeprecatedLayout to wideLayout
• add css hack to remove frontmatter from showing up on .mdx pages
• remove hideTopButton from frontmatter
• have Layout to wrap all components by moving to _app.tsx with conditions
• add Component properties displayName, wideLayout, useHeader to pass to customize Layout in _app.tsx
• update getPostData function with unified - may not need this
• add TODO in Layout to remove frontMatter prop which may be unused now that next-mdx-enhanced is removed