Fix out of bounds error in "engineering" exponent mode

[joneuhauser]

In 0.3.0,

#num(0.1, exponent-mode: "engineering")

breaks with an out of bounds exception.

Longer example with relevant exponent range:

// This line is optional, but it makes formatting of the positive exponents nicer  
// #metro-setup(drop-zero-decimal: true)

#let fn(number) = (
  [#num(number)], 
  [#num(number, exponent-mode: "engineering")],
  [#num(number, exponent-mode: "scientific")],
  [#num(number, exponent-mode: "threshold")],
)

#table(columns: 4,
table.header([Normal], [Engineering], [Scientific], [Threshold]), 
..(for exp in range(-6, 6) {
  fn(1 * calc.pow(10, exp))
})
)