command word: non-executable file should be unknown-token

[danielshahaf]

The following:

touch foo
chmod 0644 foo
./foo

highlights ./foo as path, but should highlight it as unknown-token (unless there happens to be an alias or function literally called ./foo).

Note that we already call stat once (since ./does-not-exist is highlighted as unknown-token).

[danielshahaf]

_zsh_highlight_main_highlighter_check_path needs to know whether the current word is at command word position, and only require check executability when at command word.

[danielshahaf]

Directories may be the command word when the AUTO_CD option is set. (However, a patch that fixes the issue only for the common case of NO_AUTO_CD would be accepted.)

[danielshahaf]

In unset x; $x ls, the ls is also a command word.

[danielshahaf]

In unset x; $x ls, the ls is also a command word.

PR #667.

[danielshahaf]

... and PR #669 for the other issues here.