parser: improve long string when mixing " and {

Signed-off-by: Jean Privat <jean@pryen.org>
parser: regenerate table for long strings

Signed-off-by: Jean Privat <jean@pryen.org>
tests: add test_string_triple2.nit

for complex mixing of { and "

Signed-off-by: Jean Privat <jean@pryen.org>
Merge branch 'long_string'

fix long strings with alternation of " and {

example of now accepted strings

~~~
a = """<field="{{{value}}}">"""
~~~

However, I am not sure how existing colorers (see the misc/ directory)
behave.

reported-by: Alexis Laferrière <alexis.laf@xymus.net>