Add `hasPrefix` and `hasSuffix` string filters

[markst]

It would be helpful to support hasPrefix and hasSuffix filters in Stencil templates for cleaner string matching. For example:

{% if style.varName|hasPrefix:"ios" %}

Currently, this requires verbose workarounds using split/join, which are less readable and unintuitive for common cases like filtering prefixed style names.

extension Font {
  struct DesignSystem {
{%- for style in styles %}
{%- if style.varName|split:"ios"|join:"" != style.varName %}
    static func {{ style.varName }}() -> FigmaFont {