fix(build): separate vanilla build

[dai-shi]

In v4, we will allow useStore + vanilla store usage, it's better to avoid code duplication.
This is how we do in valtio.

(Just noticed, UMD is not supported by use-sync-external-store. Same for valtio.)

[codesandbox-ci]

This pull request is automatically built and testable in CodeSandbox.

To see build info of the built libraries, click here or the icon next to each commit SHA.

Latest deployment of this branch, based on commit 95c613a:

Sandbox	Source
React	Configuration
React Typescript	Configuration
React Browserify	Configuration
React Snowpack	Configuration
React Parcel	Configuration
Next.js	Configuration

[github-actions]

Size Change: -547 B (-2%)

Total Size: 26.5 kB

Filename	Size	Change
dist/esm/index.js	355 B	-203 B (-36%)	🎉
dist/index.js	594 B	-67 B (-10%)	👏
dist/system/index.development.js	534 B	-132 B (-20%)	🎉
dist/system/index.production.js	358 B	-72 B (-17%)	👏
dist/umd/index.development.js	737 B	-65 B (-8%)	✅
dist/umd/index.production.js	530 B	-8 B (-1%)

ℹ️ View Unchanged

Filename	Size
dist/context.js	518 B
dist/esm/context.js	448 B
dist/esm/middleware.js	2.93 kB
dist/esm/shallow.js	272 B
dist/esm/vanilla.js	353 B
dist/middleware.js	2.99 kB
dist/shallow.js	318 B
dist/system/context.development.js	572 B
dist/system/context.production.js	393 B
dist/system/middleware.development.js	3.02 kB
dist/system/middleware.production.js	1.95 kB
dist/system/shallow.development.js	338 B
dist/system/shallow.production.js	253 B
dist/system/vanilla.development.js	421 B
dist/system/vanilla.production.js	263 B
dist/umd/context.development.js	662 B
dist/umd/context.production.js	488 B
dist/umd/middleware.development.js	3.11 kB
dist/umd/middleware.production.js	1.93 kB
dist/umd/shallow.development.js	447 B
dist/umd/shallow.production.js	344 B
dist/umd/vanilla.development.js	562 B
dist/umd/vanilla.production.js	364 B
dist/vanilla.js	424 B

_{compressed-size-action}