`args[numArgs - 1]` is the last argument in the list, so correct usage was being flagged as incorrect, and incorrect usage wasn't working anyway because the filter was `"undefined"`.
Additionally, the function seemed confused as to whether or not `ls` counted as an argument (AFAICT, it isn't).
Fixes#1994.
Found most (hopefully all) places where the error message wording
incorrectly was shown to the tune of 'no script exists', where it should
have been showing to the effect of 'script is not running'.
Also cleaned up some of the consistency in the wording and added a
'helper' export for knowing valid script extensions used in validation
of 'isScriptFilename', so we can have consistent error messaging.
Resolvesdanielyxie/bitburner#1966
Cat ends up translating the path it receives from relative to absolute twice, which I fix by changing the filename to an absolute path before it's passed to getTextFile with a leading "/" so that it doesn't interpret the filename as being relative.
Read I fixed by causing getTextFile to remove the leading "/" from files that are in the root directory, since that is required to translate their name into the native "filesystem"s format.
For some time now, filenames have included the extension, so there's no need to append ".js" either on a single-file download or zip download.
While I was in the area, I also fixed .txt files in a folder - they had the same bug as scripts used to (appear in the zip as an underscore). I consolidated the code since it was the same for script files and text files.
I also added the ability to download *.js and *.ns (previously only supported *, *.script, and *.txt)
I also made the wildcard matching just a bit more flexible, so if someone really wanted to, they could go "download *blah.js" and get all scripts ending with that pattern ¯\_(ツ)_/¯
