[ANN] Desmos.jl

One bothersome approach is creating DesmosState instances manually.
I think useful non-macro API should be provided, but the design is not decided yet.

# Export `DesmosState` and `desmos_latexify`.
using Desmos

# These are part of stable API, but not exported.
using Desmos: DesmosExpressions, DesmosExpression

# This structure is the same as the Desmos JSON format.
DesmosState(
    expressions = DesmosExpressions(
        list = [
            DesmosExpression(
                id = string(i),
                latex = desmos_latexify(:(sin($i*x)/$i))
            ) for i in 1:10
        ]
    )
)