Automatically wrap JSX string literals with Jsx.string()

[Copilot]

This PR implements automatic wrapping of string literals in JSX children with Jsx.string() calls, eliminating the need for manual wrapping.

Problem

Currently, string literals in JSX children must be explicitly wrapped:

// Current - requires manual wrapping
<span>{Jsx.string("hello")}</span>
<> {Jsx.string("world")} </>

// What developers want to write
<span>hello</span>
<> world </>

Solution

Modified the JSX children parser in compiler/syntax/src/res_core.ml to automatically detect string literals and wrap them with Jsx.string() calls during AST construction.

Implementation Details

1. Added wrap_jsx_string_literal function that:

   • Detects string literal expressions (Pexp_constant (Pconst_string ...))
   • Creates AST nodes representing Jsx.string(originalString) function calls
   • Preserves original source location information

2. Integrated into parse_jsx_children:

   • Applied to both JSX parsing paths (element and fragment children)
   • Only affects direct string literals, not expressions in braces
   • Maintains backward compatibility with existing code

Examples

After this change, the following transformations happen automatically:

// Input
<span>hello world</span>
// Parsed as equivalent to
<span>{Jsx.string("hello world")}</span>

// Input  
<> fragment text </>
// Parsed as equivalent to
<> {Jsx.string("fragment text")} </>

// Input
<div>first<span>nested</span>last</div>
// Parsed as equivalent to  
<div>{Jsx.string("first")}<span>{Jsx.string("nested")}</span>{Jsx.string("last")}</div>

Backward Compatibility

• Existing explicitly wrapped strings continue to work unchanged: <span>{Jsx.string("hello")}</span>
• Expressions in braces remain unaffected: <span>{someVariable}</span>
• Only bare string literals get automatically wrapped

Test Cases

Added comprehensive test cases to tests/syntax_tests/data/parsing/grammar/expressions/jsx.res covering:

• Basic string literals in JSX elements
• String literals in JSX fragments
• Multiple string literals with nested elements
• Mixed content scenarios

The implementation follows established patterns in the ReScript compiler codebase and maintains full compatibility with existing JSX usage patterns.

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[nojaf]

@tsnobip I have doing some work in this area over in https://github.com/rescript-lang/rescript/compare/master...nojaf:rescript:jsx2

[tsnobip]

@nojaf nice! The handling of whitespace inside HTML/JSX is such a mess though, good luck with that! I'll then focus on wrapping variables inside curly braces for now then. This way, users of V12 will have their projects automatically formatted and we can introduce your work in v12.1 (or as soon as it's ready!)!