fix(opfunc): fix linewise comment for partial visual selection in single line (Bug#476)

[omeyenburg]

This PR addresses issue #476 where creating a linewise comment inadvertently produces a blockwise comment under certain conditions, specifically in VISUAL mode when a portion of a single line is selected.

When selecting a part of a single line in VISUAL mode, such as the printf in the following code:

int main() {
    printf("Hello World");
}

Pressing either gc or gb currently results in:

int main() {
    /* printf */("Hello World");
}

For a linewise comment using gc, the expected result should be:

int main() {
    // printf("Hello World");
}

This PR fixes this problem both for the shortcut (gc) and the api command (require'Comment.api'.toggle.linewise.current()). Explicitly using a blockwise comment (e.g. using gb) still works.