meson: restore extension_memory as feature (fixes #829)

[adityachilka1]

Fixes #829.

The bug

meson setup fails on current master:

meson.build:45:16: ERROR: Unknown method "enabled" in object
<[BooleanHolder] holds [bool]: False> of type BooleanHolder.

Root cause

Commit 29451d6c changed extension_memory from type: 'feature' to
type: 'boolean' - but that branch was PR #764 ("Fix: update meson options
to avoid deprecation warnings"), whose actual diff just removed quotes from
boolean values (Meson 1.1 deprecation fix). The PR branch was based on an
older master where extension_memory was still boolean. When the PR was
merged into master (commit 6502a671), the conflict resolution kept the
PR's stale boolean version instead of master's current feature version.

meson.build still calls build_memory.enabled() and build_memory.auto(),
which are feature-option methods; on a boolean option they don't exist.

The fix

Restore extension_memory to its feature type in meson_options.txt:

-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.')

meson.build is UNCHANGED - its use of enabled() and auto() is
correct for a feature option and preserves the original "memory extension
automatically enabled when fixture is enabled" semantic.

Verification

meson setup build                        # OK (pre-fix: ERROR)
meson configure build -Dextension_memory=enabled  # OK
ninja -C build                           # builds libunity.a + memory

Dropping the fix (git stash) reproduces the original error verbatim.

No user-facing change

Users who were explicitly passing -Dextension_memory=false can now use
-Dextension_memory=disabled (feature semantic). The default ('auto')
matches the pre-breakage default, so unconfigured builds are unchanged.

(Credit to the issue reporter for diagnosing the accidental type change.)