feat(jekyll): add conditional increment plugin "inc"

[bartsmykla]

This new inc plugin enables conditional incrementation of values based on version checks, addressing a limitation in Jekyll's native {% increment %} tag, which increments unconditionally, even within {% if_version ... %} blocks.

With this plugin, we can now increment values only when specific version conditions are met, as shown:

## Step {% inc step_number if_version="gte:2.9.x" %}: ...

or

[Link to other page's Step {% inc guide_step if_version="lte:2.4.x" init_value=5 %}: ...](...)

Additionally, I refactored parameter parsing logic into a separate module, making it reusable for other plugins.

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