Package-provided default cache settings per function

[lkdvos]

Closes #19.

Adds an overloadable hook, MemoizationKit.default_cache_settings(f), so a package can ship default cache settings for its own functions without writing to the user's preferences or overriding them from __init__:

MemoizationKit.default_cache_settings(::typeof(device_treebraider)) = (; measure = "bytes", maxsize = 2^28)

Resolution order:

set_cache_size! > [<Pkg>.MemoizationKit.<f>] > [<Pkg>.MemoizationKit] > default_cache_settings(f) > [MemoizationKit] > built-in

Package defaults take precedence over the global [MemoizationKit] section, so a generic user limit does not override a smaller limit a package chose (e.g. for device memory), while package- and function-specific preferences still do.

Open points from the issue:

• Precedence: placed above the global section, as discussed above.
• Package-wide hook: not added. Module values can't be distinguished by dispatch, so default_cache_settings(::Module) couldn't be specialized per package; a union method such as ::Union{typeof(f), typeof(g)} covers several functions instead.
• Extension timing: settings are read when the first cache is created, as with preferences; this is documented.

Other changes:

• The SQLite extension uses the same resolution, so the hook also applies to disk and disk_path.
• Unknown or invalid package defaults throw an ArgumentError on the first call, since they are a bug in the package; invalid preferences are still ignored with a warning, whose text now says "setting" instead of "preference".
• Documentation added in the configuration guide and the API reference.

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