Make SPIFFS garbage collection publicly available (#5944)

Original issue: https://github.com/esp8266/Arduino/issues/2870

cores/esp8266/FS.cpp

@@ -262,6 +262,13 @@ void FS::end() {
     }
 }
 
+bool FS::gc() {
+    if (!_impl) {
+        return false;
+    }
+    return _impl->gc();
+}
+
 bool FS::format() {
     if (!_impl) {
         return false;

cores/esp8266/FS.h

@@ -206,6 +206,8 @@ public:
     bool rmdir(const char* path);
     bool rmdir(const String& path);
 
+    bool gc();
+
 protected:
     FSImplPtr _impl;
 };

cores/esp8266/FSImpl.h

@@ -82,6 +82,7 @@ public:
     virtual bool remove(const char* path) = 0;
     virtual bool mkdir(const char* path) = 0;
     virtual bool rmdir(const char* path) = 0;
+    virtual bool gc() { return true; } // May not be implemented in all file systems.
 };
 
 } // namespace fs

cores/esp8266/spiffs_api.h

@@ -208,6 +208,11 @@ public:
         return true;
     }
 
+    bool gc() override
+    {
+        return SPIFFS_gc_quick( &_fs, 0 ) == SPIFFS_OK;
+    }
+
 protected:
     friend class SPIFFSFileImpl;
     friend class SPIFFSDirImpl;