feat($interpolate): escape interpolated expressions

the default escaped interpolation signs are `{{{{` and `}}}}`. those symbols will be ignored when parsing the interpolated string and will only be replaced by `{{` and `}}` in the result string. this allows servers that put unsafe strings inside html templates to replace `{{` with `{{{{` and optionally `}}` with `}}}}` in order to prevent XSS attacks.

Closes #5601