feat(styles): added flexible customization of indents in lists

[makhnatkin]

This PR modifies src/scss/_common.scss to add CSS variables, enabling customizable margins for consistent Markdown list styling.

PostCSS settings

The build was updated in postcssPresetEnv to use this configuration:

postcssPresetEnv({
    stage: 0,
    features: {
        'logical-properties-and-values': false,
    },
}),

This disables logical properties processing (see postcss-logical) to preserve the original variable values.

New CSS Variables

The table below lists the four new CSS variables, their purposes, and default values:

Variable Name	Purpose	Default Value
--yfm-text-block-margin-block	Sets vertical margins for text blocks (e.g., p, blockquote, ul, ol, dl, table, pre)	0 15px
--yfm-list-item-margin-block	Defines vertical margins between list items (li + li)	0.33em 0
--yfm-list-text-margin-block	Controls vertical margins for text elements (p, blockquote) within list items	15px
--yfm-list-text-last-margin-block	Adjusts vertical margins for the last text element (p, blockquote) in a list item	15px

Customization Example

These variables can be overridden in CSS for flexibility. It ensures uniform rendering and removes the bottom margin of the last paragraph in list items. Fixes #649

:root {
    --yfm-text-block-margin-block: 15px 0;
    --yfm-list-text-last-margin-block: 0;
}

Before

After