FontCache: Default destructor within the cpp file

Avoids potentially inlining a lot of repeated destruction logic. This
also allows the class to play nicer with forward declarations.

While we're at it, we can also explicitly delete the move
assignment/constructor (previously they were implicitly deleted, given
the class contains a const reference).

specter/include/specter/FontCache.hpp

@@ -176,6 +176,8 @@ class FontCache {
 
 public:
   FontCache(const hecl::Runtime::FileStoreManager& fileMgr);
+  ~FontCache();
+
   FontCache(const FontCache& other) = delete;
   FontCache& operator=(const FontCache& other) = delete;
 

specter/lib/FontCache.cpp

@@ -613,6 +613,8 @@ FontCache::FontCache(const hecl::Runtime::FileStoreManager& fileMgr)
   hecl::MakeDir(m_cacheRoot.c_str());
 }
 
+FontCache::~FontCache() = default;
+
 FontTag FontCache::prepCustomFont(std::string_view name, FT_Face face, FCharFilter filter, bool subpixel, float points,
                                   uint32_t dpi) {
   /* Quick validation */