Add GreaterThanOrEqual and LessThanOrEqual methods

[pd93]

Simple PR to add LessThanOrEqual and GreaterThanOrEqual methods. These are syntactically the same as the existing LessThan, GreaterThan and Equal methods in that they are just wrappers around Compare.

The main motivation for this is code readability. I have several places in my projects where I am doing something like this:

const v3 = semver.MustParse("3")
...
if version.Compare(v3) >= 0 {
    // do something
}

it would be much more readable to use these new wrappers:

const v3 = semver.MustParse("3")
...
if version.GreaterThanOrEqual(v3) {
    // do something
}