feat(repl): add font size adjustment to editor

[ssssota]

Added a font size change feature to the REPL editor. This is necessary to make the code more readable when screen sharing or projecting the Playground for Svelte promotion.
(Since there was no discussion in the issue, it's okay to close it if it doesn't meet your intentions.)

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[7nik]

The standard page zoom should work much better for this purpose: it scales everything, including the result section (and other tabs), docs, etc. How is this better?

[ssssota]

Thank you for your prompt confirmation!

With page zoom, the header is also enlarged, which can cause the header to become too large when projected.
Given that general code editors are equipped with font size adjustment features in addition to zoom functionality, I believe there is a certain validity to enlarging only the code's font.