move matrix and hierCluster code to plots/matrix subdir

[siosonel]

Description

First round of matrix code reorganization:

• moved matrix code to plots/matrix subdir
• use subpath alias as appropriate to import dom, shared, common client code
• create index files with transitive exports to simplify subpath alias

Next rounds will split large code files like matrix.interactivity.js into smaller files, and also migrate to .ts file as appropriate.

Tested with:

• http://localhost:3000/testrun.html?name=*.unit
• cd client && npm run test:integration
• local GFF dev environment: oncomatrix and gene exp clustering still works

Checklist

Check each task that has been performed or verified to be not applicable.

• Tests: added and/or passed unit and integration tests, or N/A
• Todos: commented or documented, or N/A
• Notable Changes: updated release.txt, prefixed a commit message with "fix:" or "feat:", added to an internal tracking document, or N/A