Problem:
'isident' decides how env var names are parsed, which is more
"confusing" than "useful":
- Changing 'isident' (for 'define' or "\i") can break `$NAME` expansion.
- Its default byte ranges let multibyte names work on Windows (`$aあ`)
but not Unix (`$é`).
Solution:
Ignore 'isident' when parsing env var names.