Add Animated Gifs support

[Tawank]

Proposed Changes

• added gif support
• added loading boot.gif animation on boot
• added drawGif, gifOpen, gifPlayFrame, gifDimensions, gifReset, gifClose, getFreeHeapSize, setCursor, print and println functions to js interpreter
• added js script and gifs to sd_files
• in js interpreter: changed print function witch prints to serial to serialPrintln as there is serialReadln
• changed tft.fillRect(0,0,tftWidth,tftHeight,COLOR) to tft.fillScreen(COLOR) as it is faster

Types of Changes

Bugfix, New Feature

Verification

You can run script gifs.js.
Set boot animation.
View gifs with file explorer.

User-Facing Change

- added gif support
- added loading boot.gif animation on boot
- added `drawGif`, `gifOpen`, `gifPlayFrame`, `gifDimensions`, `gifReset`, `gifClose`, `getFreeHeapSize`, `setCursor`, `print` and `println` functions to js interpreter
- added js script and gifs to sd_files
- in js interpreter: changed `print` function witch prints to serial to `serialPrintln` as there is `serialReadln`
- 

Further Comments

https://www.youtube.com/watch?v=EiVVoIyECkQ
I tested drawing 4 gifs simultaneously.