Remove spacing of spaced symbols/commands if not used as binary operator

[manuelbb-upb]

Tackle one of the problems in #61, i.e., spacing around symbols that by default represent binary operators but can have unary meaning too.
Old tests succeed even with this heuristic enabled; there are some new tests.

using GLMakie
using LaTeXStrings
import MathTeXEngine as MTE
Z = rand(2, 10)

function test_fig(unspace=true)
    global Z
    MTE.UNSPACE_BINARY_OPERATORS_HEURISTIC[] = unspace
    fig = Figure(; size=(480,480))
    Label(fig[0,1], "unspace=$(unspace)"; fontsize=10)
    s=L"$-1, 2-1, (-1), (2-1)+1$\\\\$\frac{1}{2}\pm\sqrt{3}\mp 1$\\\\$\frac{1}{2}{}\pm\sqrt{3}{}\mp 1$\\\\$ℯ^{-1},ℯ^{2-1}ℯ^\pm$"
    Label(fig[1,1], s; fontsize=40)
    
    return fig    
end

fig1 = test_fig(true)
save(joinpath(@__DIR__, "fig_unspaced.png"), fig1)
fig2 = test_fig(false)
save(joinpath(@__DIR__, "fig_spaced.png"), fig2)