warn for possible misname component to lowercase

[tanhauhau]

• if the element tag name is a variable name within the scope of the component, it may be using it as lowercase unintentionally. we should warn the user as described in No mention that components must be named starting with an uppercase letter #4206 (comment)

eg:

<script>
  import thisShouldWarnMe from './MyComponent.svelte';
</script>

<thisShouldWarnMe />

Before submitting the PR, please make sure you do the following

• It's really useful if your PR references an issue where it is discussed ahead of time. In many cases, features are absent for a reason. For large changes, please create an RFC: https://github.com/sveltejs/rfcs
• This message body should clearly illustrate what problems it solves.
• Ideally, include a test that fails without this PR but passes with it.

Tests

• Run the tests with npm test and lint the project with npm run lint

[Conduitry]

I'm a little concerned about false positives, like

<script>
	let i;
</script>

<i>Hello world</i>

Is there a reasonable way to better characterize the situations where a mistake is likely being made? Maybe only if the variable in question is an import?

[tanhauhau]

@Conduitry updated 👍

[Conduitry]

This warning is now included in 3.29.0.