Hi, thanks for this wonderful tool!

Description

The API ValidateLicenses is used to determine if any of the provided license expressions is invalid.

There's a slight issue with the casing of license expression operators in this API.

Case 1:

valid, invalidLicenses := ValidateLicenses([]string{"MIT AND APACHE-2.0"})
assert.True(valid) // works

Case 2:

valid, invalidLicenses := ValidateLicenses([]string{"MIT and APACHE-2.0"})
assert.True(valid) // doesn't work

According to the SPDX specification, "License expression operators (AND, and, OR, or, WITH and with) should be matched in a case-sensitive manner, i.e., letters must be all upper case or all lower case." Ref: https://spdx.github.io/spdx-spec/v3.0.1/annexes/spdx-license-expressions/.
Thus, Case 2 should have been valid too.

Desired Behaviour

The API ValidateLicenses accepts license expressions with all lowercase operators too(and, or, with).