Add @mustuse (DIP 1038) to the language spec

[pbackus]

DIP: https://github.com/dlang/DIPs/blob/master/DIPs/accepted/DIP1038.md
DMD PR: dlang/dmd#13589.

[dlang-bot]

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[pbackus]

Failed to download package eventcore from https://code.dlang.org/packages/eventcore/0.9.7.zip

Force-pushing to retry.

[pbackus]

Now lower case; see dlang/druntime#3731 for details.

[pbackus]

Ping @dkorpel, DMD PR is merged so we can move ahead with this one now.