Closes zed-industries#20036

This introduces new action `editor: insert snippet`. It supports three
modes:

```
["editor::InsertSnippet", {"name": "snippet_name"}]
["editor::InsertSnippet", {"language": "language_name", "name": "snippet_name"}]
["editor::InsertSnippet", {"snippet": "snippet with $1 tab stops"}]
```

## Example usage

### `keymap.json`

```json
  {
    "context": "Editor",
    "bindings": {
      // named global snippet
      "cmd-k cmd-r": ["editor::InsertSnippet", {"name": "all rights reserved"}],
      // named language-specific snippet
      "cmd-k cmd-p": ["editor::InsertSnippet", {"language": "rust", "name": "debug-print a value"}],
      // inline snippet
      "cmd-k cmd-e": ["editor::InsertSnippet", {"snippet": "println!(\"This snippet has multiple lines.\")\nprintln!(\"It belongs to $1 and is very $2.\")"}],
    },
  },
```

### `~/.config/zed/snippets/rust.json`

```json
{
  "debug-print a value": {
    "body": "println!(\"$1 = {:?}\", $1)",
  },
}
```

### `~/.config/zed/snippets/snippets.json`

```json
{
  "all rights reserved": {
    "body": "Copyright © ${1:2025} ${2:your name}. All rights reserved.",
  },
}
```

## Future extensions

- Support multiline inline snippets using an array of strings using
something similar to `ListOrDirect` in
`snippet_provider::format::VsCodeSnippet`
- When called with no arguments, open a modal to select a snippet to
insert

## Release notes

Release Notes:

- Added `editor::InsertSnippet` action

Closes zed-industries#20036

This introduces new action `editor: insert snippet`. It supports three
modes:

```
["editor::InsertSnippet", {"name": "snippet_name"}]
["editor::InsertSnippet", {"language": "language_name", "name": "snippet_name"}]
["editor::InsertSnippet", {"snippet": "snippet with $1 tab stops"}]
```

```json
  {
    "context": "Editor",
    "bindings": {
      // named global snippet
      "cmd-k cmd-r": ["editor::InsertSnippet", {"name": "all rights reserved"}],
      // named language-specific snippet
      "cmd-k cmd-p": ["editor::InsertSnippet", {"language": "rust", "name": "debug-print a value"}],
      // inline snippet
      "cmd-k cmd-e": ["editor::InsertSnippet", {"snippet": "println!(\"This snippet has multiple lines.\")\nprintln!(\"It belongs to $1 and is very $2.\")"}],
    },
  },
```

```json
{
  "debug-print a value": {
    "body": "println!(\"$1 = {:?}\", $1)",
  },
}
```

```json
{
  "all rights reserved": {
    "body": "Copyright © ${1:2025} ${2:your name}. All rights reserved.",
  },
}
```

- Support multiline inline snippets using an array of strings using
something similar to `ListOrDirect` in
`snippet_provider::format::VsCodeSnippet`
- When called with no arguments, open a modal to select a snippet to
insert

Release Notes:

- Added `editor::InsertSnippet` action