mdl 1: a log/message template containing a line break cannot be written as plain template text

[ako]

Found while implementing fmt --upgrade (#742/#735).

Under mdl 1, \\ is no longer an escape (#741), and a multi-line string literal is stored as the expression exactly as written. So a log or show message template that must contain a line break has no mdl 1 spelling as plain template text. Rewriting it turns the text into a {1} parameter, which writes a different microflow, as the execute-both test showed on mdl-examples/.../264-log-node-expression-roundtrip.mdl. That script stays on mdl 0 in the upgrade test's keepsItsVersion list.

Decide how mdl 1 spells a line break inside a template. Two options:

• a literal line break inside the quotes, stored as template text;
• $$…$$, as used for other foreign content.

Then add the fmt --upgrade rewrite and remove 264 from keepsItsVersion.