fix: invalidate values starting with 'e' + digit

[PEsteves8]

Fixes #48

Prevents values starting with 'e' + digit from being considered numbers.

Currently, if the first character is an 'e' and it's followed by a digit, containsNumber will be set to true.

This results in values that aren't units at all, being processed as such. One such example is animation names which, after hashing can be broken.
The package cssnano/postcss-convert-values, runs this line using the return value from postcss-value-parse here https://github.com/cssnano/cssnano/blob/master/packages/postcss-convert-values/src/index.js#L26

An input of e6abc would be converted to NaNabc.

The issue causes animation bugs in which a rule like:

el {
  animation: e1abc 1s linear
}

would be converted to:

el {
  animation: NaNabc 1s linear
}

The animation name reference in the keyframes declaration would remain the original one, breaking the animation.

[alexander-akait]

/cc @TrySound