Warn when requested font variant is missing and report fallback target

Description

Currently, Typst issues a warning if a requested font family name does not exist. In Typst, a specific font file corresponds to a tuple of (family-name, weight, style, stretch), which I refer to here as a "font variant".

I propose that Typst should also allow issuing a warning when a specifically requested font variant does not exist, even if the family name itself is found.

Proposed Behavior

Detection: If a user requests a specific font variant (e.g., specific weight or style) that is not available, the compiler should be able to emit a warning.

Detailed Reporting: The warning should ideally specify

• The requested variant that was missing

• The actual variant that Typst fell back to

whenever this infomation is available.

Configuration

Since the importance of strict font variant matching varies between different users and projects, this warning should ideally be configurable (e.g., allowed to be toggled on/off or suppressed by the user).

Use Case

This feature would allow users to explicitly understand when and how font fallback occurs. For example, it can help users resolve potential issues in the following two scenarios:

• Typst Code Typo: If the user accidentally specified a wrong font weight or font style in the code, the warning makes it obvious that the code needs correction.

• Missing Font Asset: If the code is correct and the specific variant was intended, the warning informs the user that they need to find the corresponding font file for that variant and ensure it is placed in a location Typst can recognize.

Related issue: #2818
Related PR: #3354

No files or tests are named in the issue. Start by reviewing the font variant resolution and warning configuration paths, then read related issue #2818 and PR #3354 for existing context; done should include configurable missing-variant warnings that report both the requested and selected fallback variants when available.