meson: restore extension_memory as feature (fixes #829)

Commit 6502a67 (merge of PR #764) accidentally downgraded the
`extension_memory` option from `type: 'feature'` to `type: 'boolean'`
during conflict resolution. The PR itself just removed quotes around
boolean values (the Meson 1.1 deprecation fix), but its branch was
based on an older master where extension_memory was still boolean;
the merge resolution chose that stale version over master's `feature`.

meson.build still calls `build_memory.enabled()` and `.auto()`, which
are feature-option methods; on a boolean option they raise:

    ERROR: Unknown method "enabled" in object
    <[BooleanHolder] holds [bool]: False> of type BooleanHolder.

Restore the `type: 'feature', value: 'auto'` declaration that existed
on master before the merge. No change needed in meson.build; its usage
of enabled() + auto() is correct for a feature option.

Verified:
  - `meson setup` now succeeds (pre-fix: failed with the error above)
  - `meson configure -Dextension_memory=enabled` applies cleanly
  - `ninja -C build` builds libunity.a including the memory extension

Author: Aditya-Chilka

meson_options.txt

@@ -1,5 +1,5 @@
 option('extension_fixture', type: 'boolean', value: false, description: 'Whether to enable the fixture extension.')
-option('extension_memory', type: 'boolean', value: false, description: 'Whether to enable the memory extension.')
+option('extension_memory', type: 'feature', value: 'auto', description: 'Whether to enable the memory extension. By default this is automatically enabled when extension_fixture is enabled.')
 option('support_double', type: 'boolean', value: false, description: 'Whether to enable double precision floating point assertions.')
 option('support_int64', type: 'boolean', value: false, description: 'Whether to enable support for 64bit integers. false means autodetect.')
 option('fixture_help_message', type: 'string', description: 'If the fixture extension is enabled, this allows a custom help message to be defined.')