Add 'src' directory to package.json files list

[HIRANO-Satoshi]

## Proposed changes
                                                                                                           
PR #654 exposed component sub-paths and added `components/` to `files`.                                                           
However, components import shared modules from `src/` (e.g. `src/component.ts`, `src/waves.ts`).                                  
This PR adds `"src"` to the `files` field in `package.json` so individual component imports resolve properly when installed as a  package.                                                                                                                            
                                                                                                                                  
## Screenshots (if appropriate) or codepen:                                                                                       
N/A (package packaging fix)                                                                                                       
                                                                                                                                  
## Types of changes                                                                                                               
- [x] Bug fix (non-breaking change which fixes an issue).                                                                         
- [ ] New feature (non-breaking change which adds functionality).                                                                 
- [ ] Breaking change (fix or feature that would cause existing functionality to change).                                         
                                                                                                                                  
## Checklist:                                                                                                                     
- [x] I have read the **[CONTRIBUTING document](https://github.com/materializecss/materialize/blob/main/CONTRIBUTING.md)**.       
- [x] My commit messages follows the conventional commit format.
- [ ] My change requires a change to the documentation, and updated it accordingly.                                               
- [ ] I have added tests to cover my changes.                                                                                     
- [ ] All new and existing tests passed.