Reject uppercase in admonition type

[almarouk]

Pull Request Description

Uppercase letters shouldn't be allowed in admonition type. Otherwise, it will cause failure when checking if the callout/admonition is a custom one; for example in the following:

admonitions/src/callout/manager.ts

Line 57 in f193899

const admonition = this.plugin.admonitions[type];

Changes Proposed

• Add an additional validation case that tests for uppercase.

Related Issues

Checklist

• I have read the contribution guidelines and code of conduct.
• I have tested the changes locally and they are working as expected.
• I have added appropriate comments and documentation for the code changes.
• My code follows the coding style and standards of this project.
• I have rebased my branch on the latest main (or master) branch.
• All tests (if applicable) have passed successfully.
• I have run linters and fixed any issues.
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