feat(gap): add itGap, itGapX, itGapY directives for CSS gap control (#580)

[giulio-leone]

Closes #580

Problem

Bootstrap Italia's .g-*, .gx-*, .gy-* classes only set CSS variables consumed by the .row class, forcing developers to use row/column wrappers for simple flex gap spacing.

Solution

Add three standalone Angular directives that apply real CSS gap properties:

Directive	CSS Property	Description
[itGap]	gap	Both axes
[itGapX]	column-gap	Horizontal axis only
[itGapY]	row-gap	Vertical axis only

All directives use Bootstrap Italia's spacer scale (0-5):

Scale	Value
0	0
1	0.25rem
2	0.5rem
3	1rem
4	1.5rem
5	3rem

Usage:

<div class="row gx-3">
  <div class="col-auto">Text 1</div>
  <div class="col-auto">Text 2</div>
</div>

<div class="d-flex" [itGapX]="3">
  <span>Text 1</span>
  <span>Text 2</span>
</div>

<div class="d-flex flex-wrap" [itGapX]="2" [itGapY]="4">...</div>

Testing

• 7 tests covering gap (4 scales), column-gap, row-gap, and combined usage
• Double gate passed: 116/116 tests ✅, 0 lint errors ✅

Checklist

• Follows CONTRIBUTING.md guidelines
• Standalone components (no NgModule)
• Selector prefix it enforced
• Exported from public_api.ts
• Conventional commit message

⚠️ This reopens #644 which was accidentally closed due to fork deletion.

[giulio-leone]

@simionato-ennovaresearch this is the active successor to #644 and it already includes the follow-up you asked for there: both the Angular-friendly directive API and the plain CSS class path, with examples/docs.

Current checks are green. If the approach looks right now, I would appreciate a review on this open PR rather than on the closed predecessor. Thanks!

[valeriocomo]

@astagi will investigate the problem reported in #580

[simionato-ennovaresearch]

@valeriocomo it wasn't really a problem, I added this comment afterwards