dillon1000/react

Distinguish JSXText/StringLiteral during codegen

Fixes an issue with incorrect spacing where spaces were getting dropped, despite 
an explicit `{" "}` in the input. The issue is that we didn't maintain JSXText 
all the way through compilation. BuildHIR distinguishes string literals (such as 
the above, inside an expressioncontainer) from JSXText, and we propagate this 
distinction all the way through to codegen. 

But then codegen stores temporary values as `t.Expression` nodes, which means we 
have to convert the JSXText nodes to StringLiteral and we lose the distinction. 
This PR updates codegen to save temporaries as `t.Expression | t.JSXText` so 
that we can preserve the difference. In most places we just coerce the value to 
an expression, but the code for emitting JSX child items looks at the raw value 
so it can distinguish them. JSXText is emitted as-is, while StringLiterals are 
always wrapped in an expression container. 

See the new test case which demonstrates the expression being preserved.