fix(env): multibyte $VAR name may end mid-char

Problem:
'isident' is checked per byte (inherited from Vim), so a name can end
in the middle of a multibyte char: with "192-255", `$é` (C3 A9) looks
up "\xC3".
- Unix: expands an env var named "\xC3", plus a stray "\xA9".
- Windows: uv_os_getenv() rejects the name (invalid UTF-8), and
  os_getenv() logs an error.

`$VAR` in expressions (get_env_len()) and cmdline completion have the
same scan.

Solution:
`vim_isIDp()`: a multibyte char is an identifier char only if it is valid
UTF-8 and 'isident' has all its bytes. Use it in expand_env_esc(),
get_env_len() and cmdline completion. Names whose bytes are all in
'isident' still work, e.g. `$aあ` with the Windows default.

Assert UV_EINVAL in os_getenv() and os_getenv_buf() (like
os_env_exists() and os_setenv() already do): Nvim no longer passes
invalid UTF-8.

0 files changed, 0 insertions, 0 deletions