Consider naming of PluralCategory string parsing functions

[sffc]

From @markusicu:

Rename method names from_tr35_… to from_uts35_… Could omit the _tr35_ infix altogether, and then just document, and maybe implement the From<> trait Could also use from_ldml_… Could also use from_cldr_… Should also change the docs for the methods accordingly Note: It used to be that everything was a TR, but then some things were elevated to a UTS. Then others were on the side - UAXes. Also, the proper way to write them is “UTS #35”. That makes it troublesome in Github markdown. And unicode.org/reports do not show it, but the title for each TR shows the proper styling of the formal title

CC @zbraniecki