Add `:tag:` Option for Custom Equation Numbering in `{math}` Directives

[mscharling]

Describe the feature you'd like to request

I am proposing the addition of a :tag: option for the {math} directive. Currently, equations can only be labeled for referencing within the document using :label:, and they automatically receive a numeric identifier. It would be beneficial, especially when covering properties of some specific math area, to allow custom tags for equations using a :tag: option.

Describe the solution you'd like

Enable the use of a :tag: option within {math} blocks to specify custom equation numbers directly in the text, allowing for alphanumeric identifiers like "A1", "B2", etc. This feature would help maintain consistency in mathematical notation, especially in texts transitioning from traditional LaTeX environments or requiring specific identification schemes for equations.

Describe alternatives you've considered

I have considered using the LaTeX \tag{} command within double-dollar environments as an alternative to address custom equation tagging needs: $$\frac{a}{b} \pm \frac{c}{d} = \frac{ad \pm bc}{bd} \tag{B1}$$ While this method allows for custom tags within equations, it introduces inconsistencies in font style between the equation content and the tag. The tag rendered by \tag{} often does not match the main document font when refering, leading to a disjointed visual appearance.

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