Update code snippets to use the platform

[TejasQ]

Why

Currently, the default greeter Tauri app when using the quickstart does not ship a valid HTML form. This is evidenced by nothing happening when pressing "Enter" after typing a name in the name input field:

This PR uses proper HTML form elements and their onsubmit handler to have the greeter actually submit a form on Enter, instead of a button receiving a click event only in isolation. I feel this is important as it teaches newcomers stuff and we want to teach them the right stuff.

Caveat

This PR doesn't touch the yew template because I don't know Rust and I'm not sure how to e.preventDefault in an onsubmit handler with Yew. We can treat Yew separately in #332.

What kind of change does this PR introduce?

• Bugfix
• Feature
• Docs
• New Binding issue #___
• Code style update
• Refactor
• Build-related changes
• Other, please describe:

Does this PR introduce a breaking change?

• Yes, and the changes were approved in issue #___
• No

Checklist

• When resolving issues, they are referenced in the PR's title (e.g fix: remove a typo, closes #___, #___)
• A change file is added if any packages will require a version bump due to this PR per the instructions in the readme.
• I have added a convincing reason for adding this feature, if necessary

Other information

[amrbashir]

Thanks! I will handle yew in another commit.