Use custom icon for .tact files in FS

[xpyctumo]

It looks a bit weird that our main files that users need to interact with are highlighted by plain text files at the .txt level.
It would be great to separate and make our own icon for our .tact files in the Web IDE.

[novusnota]

Tact filetype icons for dark and light themes, as used in the language server: https://github.com/tact-lang/tact-language-server/tree/master/icons