Highlighting injection for writeShellApplication fails with let expression

[benjamb]

When using a let expression while assigning a value to the text attribute, the entire text is just highlighted as a string:

With the let expression moved higher up, it works:

[aldoborrero]

Fixed downstream in numtide#36 — the injection query now has a variant that follows text = let … in ''…'' through the let-expression's body, so writeShellApplication with let-bound text injects bash correctly. Merged on the numtide fork.