Add inter as an alternative for sect (Proposal 14, part I)

[MDLC01]

This PR implements the first part of Proposal 14 (iteration 1) from the Symbol Proposals document. It adds inter as an alternative for sect. Eventually, sect should be deprecated, and later removed. For more context, see the discussion on Discord.

For now, there is no infrastructure to deprecate symbols in Codex, nor is there a way to deprecate a constant in Typst. This makes it impossible to deprecate sect immediately. However, since sect is a commonly used symbol, I believe the sooner we introduce its new name, the better.

Since this PR assumes the future removal of a symbol (sect) and a variant (integral.sect), and even though this was already discussed on Discord, cc. @laurmaedje.

[MDLC01]

We should probably wait for #19 to be closed before merging this PR.

[T0mstone]

I'll mention #27 here, since this PR introduces a duplicated symbol.

[laurmaedje]

With #19 merged, we can proceed with the deprecation of sect.

[MDLC01]

I marked sect as deprecated. There is no way to mark the integral.sect variant as deprecated with the current infrastructure, but I think this is fine as it is not a widely use symbol. Also, this does not remove it anyway, so nothing will break for now.