Prohibit keywords for dot access.

I ran into this real-world example:

```julia
function throw_default_branch_error(branches, cmd_succeeded)
branch_detail = if isempty(branches)
"none of DEFAULT_BRANCH_NAMES found"
else
"couldn't decide which of $branches is the default".
end
remote_detail = if cmd_succeeded
"git remote show origin didn't specify the a HEAD branch"
else
"remote show origin failed"
end
error("""
Unable to determine the default branch locally ($branch_detail).
Also unnable to determine the default branch by querying the remote ($remote_detail).
You can typically bypass this error by specifying the version manually (i.e. pass `-rev=...` to the CLI)
""")
end

end
```

```julia-repl
julia> throw_default_branch_error([], false)
"none of DEFAULT_BRANCH_NAMES found"
```

The issue is that `(;var"end"=5). end == 5` parses just fine and does not treat `end` as a keyword. Indeed keywords are accepted after `.` in all cases that I've checked. It would be a breaking/minor change to fix this. This is present in 1.6 and 1.11.