bug#70476: `**` GNU extension only works when ERE syntax is enabled

[Dylan Brotherston]

$ sed --version
sed (GNU sed) 4.9

The documentation for sed BRE syntax states:
> As a GNU extension, a postfixed regular expression can also be followed by *; 
> for example, a** is equivalent to a*
https://www.gnu.org/software/sed/manual/sed.html#BRE-syntax
But sed reports an error when ** is used in a BRE expression

$ seq 10 20 | sed 's/1**//'
sed: -e expression #1, char 7: Invalid preceding regular expression

and only allows this syntax when ERE syntax is enabled

$ seq 10 20 | sed -E 's/1**//'
0

2
3
4
5
6
7
8
9
20

grep, which also implements this GNU extension, allows ** in BRE syntax

$ seq 10 20 | grep -o '1**'
1
11
1
1
1
1
1
1
1
1
$ seq 10 20 | grep -oE '1**'
1
11
1
1
1
1
1
1
1
1

Additionally, the documentation for the `--posix` flag states
> disable all GNU extensions.
** is specifically documented as a GNU extension, but `--posix` doesn't disable 
the use of **

$ seq 10 20 | sed --posix -E 's/1**//'
0

2
3
4
5
6
7
8
9
20