[RFC] Use markdown format for integration test snapshots

[captbaritone]

Summary

This RFC proposes migrating relay_compiler_integration test snapshots from .expected to .md files with syntax highlighting for improved readability in editors and GitHub.

Motivation: Markdown Snapshots - syntax highlighting makes snapshots easier to read and review.

Changes

• Added serialize_as_markdown() to ProjectFixture with fenced code blocks
• Made --expected-extension a CLI argument in fixture-tests
• Renamed 70 integration test snapshots: .expected → .md
• Updated update-fixtures.sh to handle both formats

Example

Before:

==================================== INPUT ====================================
//- foo.js
graphql`fragment foo on User { name }`;
==================================== OUTPUT ===================================
//- __generated__/foo.graphql.ts
[TypeScript code without highlighting...]

After:

## Input

```js title="foo.js"
graphql\`fragment foo on User { name }\`;
```

## Output

```ts title="__generated__/foo.graphql.ts"
[TypeScript code with syntax highlighting...]
```

Compatibility

✅ Other fixture tests unaffected - only `relay_compiler_integration` uses markdown
✅ All 70 tests passing

Questions

1. Migrate other test suites to markdown?
2. Additional file extensions needed?

[captbaritone]

• Example of output file in "preview" mode.
• Example of output file in text mode with syntax highlighting.