[w3c/manifest] Add `navigator.install()` (PR #1175)

This is a first pass for introducing `navigator.install()`. The goal is to specify a method that allows same-origin installs (current and background documents).

This draft currently only covers same-document installs (zero parameters) and does not take manifest IDs into account yet. 

Relevant documents:
* [Web Install API Explainer](https://github.com/MicrosoftEdge/MSEdgeExplainers/blob/main/WebInstall/explainer.md#suite-of-web-apps)
* [TAG feedback](https://github.com/w3ctag/design-reviews/issues/1051)
* [Discussion about requiring manifest ID](https://docs.google.com/document/d/19dad0LnqdvEhK-3GmSaffSGHYLeM0kHQ_v4ZRNBFgWM/edit?tab=t.0#heading=h.koe6r7c5fhdg)
* [The cross-browser future of Installable Web Apps (TPAC 2023)](https://docs.google.com/document/d/1QzHTKGDxHol7KybdqY-rll1VGd1FIirvW-mtAFxPjC4/edit?tab=t.0#heading=h.5wzb241vwwm2)

Acknowledgements:
* The algorithm steps are largely inspired by the Web Share API.

---

Closes #???

This change (choose at least one, delete ones that don't apply):

* Adds new normative requirements

Implementation commitment (delete if not making normative changes):

* [ ] WebKit (https://bugs.webkit.org)
* [ ] Chromium (https://bugs.chromium.org/)
* [ ] Gecko (http://bugzilla.mozilla.org)

Commit message:

Add API for installing web applications

Person merging, please make sure that commits are squashed with one of the following as a commit message prefix:

* chore:
* editorial:
* BREAKING CHANGE:
* And use none if it's a normative change


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<a href="https://pr-preview.s3.amazonaws.com/w3c/manifest/pull/1175.html" title="Last updated on Jun 6, 2025, 8:45 PM UTC (3b2c487)">Preview</a> | <a href="https://pr-preview.s3.amazonaws.com/w3c/manifest/1175/8ff396e...3b2c487.html" title="Last updated on Jun 6, 2025, 8:45 PM UTC (3b2c487)">Diff</a>
You can view, comment on, or merge this pull request online at:

  https://github.com/w3c/manifest/pull/1175

-- Commit Summary --

  * Add API for installing the current document

-- File Changes --

    M index.html (128)

-- Patch Links --

https://github.com/w3c/manifest/pull/1175.patch
https://github.com/w3c/manifest/pull/1175.diff

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