Allow escaping $ in strings.

[AndrewKraevskii]

Some libraries have syntax highly similar to scripty syntax which makes them impossible to use. Consider example from https://data-star.dev/guide/reactive_signals#data-text

<input data-bind:foo-bar />
<div data-text="$fooBar.toUpperCase()"></div>

Pasting this into shtml file produces this error message.

---------- SCRIPT RESULT TYPE MISMATCH ----------
A script evaluated to an unexpected type.

This attribute expects to evaluate to one
of the following types:
   - string
   - int

[script_eval_not_string_or_int]
(index.shtml) layouts:7:6:
    <div data-text="$fooBar.toUpperCase()"></div>
         ^^^^^^^^^

note: value was generated from this sub-expression:
(index.shtml) layouts:7:17:
    <div data-text="$fooBar.toUpperCase()"></div>
                    ^^^^^^^^
Scripty evaluated to type: err
Error message: 'field not found'

trace:
    layout `index.shtml`,
    content `content/about.smd`.

It would be cool to have ability to escape $ in this case. Or as an alternative making it replaceabale for another character.