UFM: Add umbMemberName component (closes #22147)

[leekelleher]

Description

Fixes #22147.

Adds a new {umbMemberName:alias} UFM (Umbraco Flavored Markdown) component that resolves a Member Picker property value to the member's display name.

Why a dedicated component? {umbContentName} already imports UmbMemberItemRepository but defaults to the document entity type when the value has no type/mediaKey discriminator. A Member Picker persists a bare GUID — ambiguous without the marker to signal intent — so a separate marker is the correct design.

Value structure support

The component resolves member references regardless of how they are stored, handling each of the following (as a single value or an array):

• Bare GUID: "3f2504e0-4f89-11d3-9a0c-0305e82c3301"
• Comma-separated GUIDs: "guid1,guid2"
• UDI: umb://member/{GUID}
• Content Picker object: { type, unique } (where unique may itself be a GUID or UDI)

Names are joined by , when multiple members resolve.

Known duplication (intentional)

The observe/alias-extraction logic is structurally similar to content-name.element.ts. A shared base was considered and deferred — if a third name-resolving UFM component appears, that's the moment to extract it.

Steps to test

1. Add a Member Picker property (alias: memberPicker) to a document type.
2. Set the property's label or description to {umbMemberName:memberPicker}.
3. Open a content node, pick a member, and confirm the member's name renders in the label/description.
4. To test multi-value support, temporarily point the marker at a custom value containing two comma-separated member GUIDs and confirm both names render.

[AndyButland]

Tests out as expected @leekelleher:

I've also verified:

• Multiple members
• UDI syntax
• MNTP output

As per the other PR - looks like we have marked-ufm.test.ts used to test various components, including umbContentName. Should a test for this be added there too?

I added the "needs docs" label as this component should be documented here (and for 18).

[leekelleher]

Thanks @AndyButland! Added two parsing tests for umbMemberName to marked-ufm.test.ts (commit 1792eac).

re: docs. I'll follow-up with a docs PR.