[ENG-3892]Shiki codeblock support decorations

[ElijahAhianyo]

rx._x.code_block(
            """
print("Hello there")
print("This is a random text here")
            """,
            language="python",
            use_transformers=True,
            decorations=[
                {

                    "start": {"line": 1, "character": 0},
                    "end": {"line": 1, "character": 11},
                    "properties": {"class": 'highlighted-word'}
                }
            ]
        )

Extra fix: add plain to language literals instead of themes.

[linear]

ENG-3892 support for syntax highlighter decorators