libedit: import snapshot 2021-09-10

libedit: vendor import snapshot 2020-07-10

Import libedit 2019-09-10

Compared to current version in base:
- great improvements on the Unicode support
- full support for filename completion including quoting
which means we do not need anymore our custom addition)
- Improved readline compatiblity

Upgrading libedit has been a pain in the past, because somehow we never
managed to properly cleanup the tree in lib/libedit and each merge has always
been very painful. After years of fighting give up and refresh a merge from
scrarch properly in contrib.

Note that the switch to this version will be done in another commit.