Set props and listener in snippet_blob_edit.vue explicitely

What does this MR do?

Sets props and listener for <blob-content-edit> (in app/assets/javascripts/snippets/components/snippet_blob_edit.vue) explicitly instead of implicitly via v-bind="$attrs" and v-on="$listeners".

Also, this MR allows the component to not have content passed to it by default. This is important for the cases when the component is used in the creation of a new snippet.

NOTE: This is a ~backstage MR, hence no CHANGELOG is provided.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Closes #213059 (closed)