Deprecate escaping leading empty cells with \

[pekkaklarck]

When deprecating ignoring lines with only ... (#3107) noticed that this strange syntax is currently supported:

*** Settings ***
Documentation    WTF
\    ...    ?
\    \    ...    ?

*** Test Cases ***
Example
    Log Many    one    two
    \    ...    three
    \    \    \    \    \    ...    four

The example above would work exactly the same way if all \ characters and optionally spaces after them were removed. My guess is that we used to support ... not only in the first column when using HTML/TSV formats and in the plain text format \ is used to escape empty cells.

This escaping syntax brings no benefits and makes no sense in general. The new parser (#3076) won't support it anymore and thus we need to deprecate it in RF 3.1.2. Luckily I expect nobody to use this strange syntax.

[pekkaklarck]

This was somewhat hard to deprecate because \ is also used for escaping leading empty cells with for loops. That syntax will be deprecated in RF 3.2 in favor of closing for loops explicitly with END (#3078), but we don't want to emit warnings in RF 3.1.2 yet.

In the end decided to deprecate escaping leading empty cells only when used before .... That covers both of the usages in the original description and elsewhere leading \ ought to be an error already now.