feat: context-aware runtime

[pmmmwh]

This PR will track the work towards a fully context-aware runtime (which is way more than I've initially imagined). This should extend the supported environments of the plugin down to very old browsers (e.g. IE) and up to the latest strict-ESM builds.

Items

• Loader - ESM options, templating on block identifiers and Webpack's hot runtime (Loader - capabilities and module system detection #338)
  • Next step: swap to newer Webpack HMR APIs in a future release
• Runtime code - convert to strict-ESM compatible format, confirm compatibility with auto mode (lax CommonJS) (Client/Socket - ESM conversion #342)
• Overlay (and Sockets) - re-implement in strict-ESM down to ES5 (Overlay - convert to Strict ESM #344)
• Plugin - pass through ESM options, capability detection from RuntimeTemplate (commits in this PR)

Fixes #294
Fixes #297
Fixes #313