This PR adds support for marking options as deprecated. When a
deprecated option is used via `set_option`, a warning is emitted
(controlled by `linter.deprecated.options`).
An `OptionDeprecation` structure with a required `since` field and an
optional `text?` field is added to `OptionDecl`. Each `set_option`
elaborator (command, term, tactic, grind) calls `checkDeprecatedOption`
to emit warnings. The C++ `register_option` is updated to account for
the new field.
As a first use, `backward.eqns.nonrecursive` and
`backward.eqns.deepRecursiveSplit` are marked deprecated. Continues
earlier work done in #11096.
