This commit prevents specially crafted modelines from making the
editor execute arbitrary Kakoune commands.
By using a tabulation character as a separator, commands can be
injected in the value of the options listed in the modeline. For
example:
# kak: tabstop=2;set-option buffer pwned yes
Fixes#3735.
This commit addresses an off-by-one error that selected `modelines`+1
lines when looking for modelines in a buffer.
Note that setting `modelines` to 0 will still select one line.
Fixes#3733.
Similarly to the <semicolon> key, make it easier to write
`:execute-keys` commands by replacing <percent> with `%`.
Highlighters can keep escaping the sign when regular expressions are
not quoted, but built-in scripts that use `%` as an editing primitive
have been modified to use the named key, for clarity.
