Add test .txt file editing for License XML Editor

[swinslow]

Adding issue here per @goneall's comment at spdx/license-list-XML#1070 (comment):

Currently, in the License XML Editor in the SPDX Online Tools, I believe it is only possible to edit the XML file. But the workflow for adding new licenses requires two files for each license:

• the XML file (e.g., /src/BSD-3-Clause.xml)
• a plain text file as a test to match against (e.g., /test/simpleTestForGenerator/BSD-3-Clause.txt)

For the License XML Editor to be a standalone tool -- in other words, to enable folks to add licenses without needing to go through the git command-line workflow -- it would be great if it could also enable adding / editing the .txt file corresponding to the .xml file.

[jlovejoy]

duplicate to newer #399 - so closing this one