Remove Heredocs, they slow down the parser

Heredocs make the parser non-deterministic and make it impossible to
make efficient parsers due to the need to store the end character, which
can't be efficiently implemented in the parser.
To make it easier to switch to a recursive ascent parser like nom, it is
better to avoid heredocs in the syntax

From a user point-of-view, heredocs are redundant and/or favourise
non-ergonomic behaviour. If the string is simple, herestring are
sufficient and do not require a user-defined terminator. If the string
is long or implies nested comments and/or code, it is probably better to
store it in a separate file and use cat for script legibility.

This has already been made by fish and does not seem like a pain point.
See https://fishshell.com/docs/current/design.html#ortho