fix: dev server watch mode

[huynhicode]

:bookmark_tabs: Summary

This PR fixes the dev server watch mode.

Currently, to see local changes to the mermaid source code (localhost:9000), you need to:

1. run pnpm dev to start the server
2. make changes to the respective file in mermaid/packages/src
3. run pnpm build separately to rebuild the mermaid.js file
4. refresh localhost:9000

Setting core to false for watch mode allows you to:

1. run pnpm dev to start the server
2. make changes to the respective file in mermaid/packages/src
3. refresh localhost:9000

Resolves #

No issue - see https://mermaid-talk.slack.com/archives/CL256P19N/p1670524518664389

:straight_ruler: Design Decisions

Describe the way your implementation works or what design decisions you made if applicable.

:clipboard: Tasks

Make sure you

• [x] :book: have read the contribution guidelines
• [ ] :computer: have added unit/e2e tests (if appropriate)
• [ ] :bookmark: targeted develop branch