Issue title: Inline conditionals
 Issue body: ### Describe the problem

The following is the code to conditionally show a single element:

```svelte
{#if show}
  <div>...</div>
{/if}
```

### Describe the proposed solution

Syntactical sugar suggestion:

**Proposal 1** — After opening tag with a space and a `?` character.
```svelte
<!-- using the variable name -->
<div ?show>...</div>

<!-- using a JS expression -->
<div ?{show === true}>...</div>

<!-- use with components -->
<SomeComponent ?show />
<SomeComponent ?{show === true} />
```

**Proposal 2** — After opening tag without a space and with a `:` character.
```svelte
<!-- using the variable name -->
<div:show>...</div>

<!-- using a JS expression -->
<div:{show === true}>...</div>

<!-- use with components -->
<SomeComponent:show />
<SomeComponent:{show === true} />
```

Common example I encounter:
```svelte
<!-- before -->
{#if subtitle}
  <p>{subtitle}</p>
{/if}

<!-- proposed solution -->
<p ?subtitle>{subtitle}</p>

<!-- too much sugar...? (variable is used as the default <slot>, if it exists) -->
<p ?subtitle />
<p:subtitle />
<Subtitle ?subtitle />
<Subtitle:subtitle />
```

### Alternatives considered

The following is done in a single line but I prefer to keep `{#}` expressions on their own line.

```svelte
{#if show}<div>...</div>{/if}
```

### Importance

nice to have
 Comments: 
 Comment 0: There have been a number of other similar issues requesting various bits of syntactical sugar, and the general answer to these has been "no", both because there shouldn't be multiple ways to do something, and because code is read many more times than it is written. Somewhere someone down the line is going to appreciate having the word `if` in the code rather than some opaque punctuation.
 Comment 1: Additional proposal (somewhere down the linedown the line...):

With an `if` directive:
```svelte
```
{'source': 'GithubIssues', 'repo_name_with_owner': 'sveltejs/svelte', 'issue_id': 1320250646, 'repo_stars': 65257.0}