proposed spelling fixes for maintain.texi, standards.texi

[Paul Eggert]

Javascript -> JavaScript (this is the accepted name for the language; 
see, for example, Internet RFC 9239)

non-existent -> nonexistent (the usual spelling in American English; 
see, for example, <https://www.merriam-webster.com/dictionary/nonexistent>).

These changes come from changes originally proposed for Gnulib's 
downstream copies of the GNU standards.

standards.diff
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