[Bug] Cannot import v5 theme if using esbuild

### Version

6.0.0

### Link to Minimal Reproduction

https://codesandbox.io/p/devbox/xp6t44

### Steps to Reproduce

1. Open the linked CodeSandbox
2. Open a new Terminal
3. Run `node build.js`

### Current Behavior

Attempting the build results in an import error:

```
Import from "echarts/theme/v5.js" to get the file "node_modules/echarts/theme/v5.js":

index.js:1:24:
1 │ import "echarts/theme/v5";
│ ^
╵ .js

You can mark the path "echarts/theme/v5" as external to exclude it from the bundle, which will
remove this error and leave the unresolved path in the bundle.

1 error
/project/workspace/node_modules/esbuild/lib/main.js:1477
let error = new Error(text);
^

Error: Build failed with 1 error:
index.js:1:7: ERROR: Could not resolve "echarts/theme/v5"
at failureErrorWithLog (/project/workspace/node_modules/esbuild/lib/main.js:1477:15)
at /project/workspace/node_modules/esbuild/lib/main.js:946:25
at /project/workspace/node_modules/esbuild/lib/main.js:898:52
at buildResponseToResult (/project/workspace/node_modules/esbuild/lib/main.js:944:7)
at /project/workspace/node_modules/esbuild/lib/main.js:971:16
at responseCallbacks. (/project/workspace/node_modules/esbuild/lib/main.js:623:9)
at handleIncomingPacket (/project/workspace/node_modules/esbuild/lib/main.js:678:12)
at Socket.readFromStdout (/project/workspace/node_modules/esbuild/lib/main.js:601:7)
at Socket.emit (node:events:518:28)
at addChunk (node:internal/streams/readable:559:12) {
errors: [Getter/Setter],
warnings: [Getter/Setter]
}
```

### Expected Behavior

I would expect the build to work! It works in other build tools, like Rspack. Changing the import path to `"echarts/theme/v5.js";` fixes the error in esbuild.

### Environment

```markdown

```

### Any additional comments?

I believe this is because of the `export` configuration in `package.json`. The `theme` wildcards are missing `.js`, I think the export needs to be changed to ` "./theme/*": "./theme/*.js",`. Strangely, this fails in esbuild, but not in other build tools like Rspack. This is not a major issue and has a simple workaround, but I thought it'd be good to report this. Thank you!