fix: exclude system message from unread count

[khushal87]

CLA

• I have signed the Stream CLA (required).
• Code changes are tested

Description of the changes, What, Why, and How?

Reference - https://stream-io.atlassian.net/browse/CRNS-406
Reference Issue - #739

Changelog

Files changed are:

• src/channel.ts
• test/unit/channel.js

[github-actions]

Size Change: +64 B (0%)

Total Size: 250 kB

Filename	Size	Change
dist/browser.es.js	54.6 kB	+11 B (0%)
dist/browser.full-bundle.min.js	30.1 kB	+15 B (0%)
dist/browser.js	55.3 kB	+14 B (0%)
dist/index.es.js	54.7 kB	+11 B (0%)
dist/index.js	55.3 kB	+13 B (0%)

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