Update the developer docs to reflect the use of JuliaSyntax.jl (#59300)

[The "parsing" section of the "eval" dev docs
page](https://docs.julialang.org/en/v1/devdocs/eval/#dev-parsing) was
not updated when JuliaSyntax.jl was adopted in 1.10. This updates the
text to reflect that it is the default parser and briefly mentions how
to switch back.

(cherry picked from commit ec27274)

Author: ajwheeler

doc/src/devdocs/eval.md

@@ -62,25 +62,11 @@ The 10,000 foot view of the whole process is as follows:
 
 ## [Parsing](@id dev-parsing)
 
-The Julia parser is a small lisp program written in femtolisp, the source-code for which is distributed
-inside Julia in [src/flisp](https://github.com/JuliaLang/julia/tree/master/src/flisp).
-
-The interface functions for this are primarily defined in [`jlfrontend.scm`](https://github.com/JuliaLang/julia/blob/master/src/jlfrontend.scm).
-The code in [`ast.c`](https://github.com/JuliaLang/julia/blob/master/src/ast.c) handles this handoff
-on the Julia side.
-
-The other relevant files at this stage are [`julia-parser.scm`](https://github.com/JuliaLang/julia/blob/master/src/julia-parser.scm),
-which handles tokenizing Julia code and turning it into an AST, and [`julia-syntax.scm`](https://github.com/JuliaLang/julia/blob/master/src/julia-syntax.scm),
-which handles transforming complex AST representations into simpler, "lowered" AST representations
-which are more suitable for analysis and execution.
-
-If you want to test the parser without re-building Julia in its entirety, you can run the frontend
-on its own as follows:
-
-    $ cd src
-    $ flisp/flisp
-    > (load "jlfrontend.scm")
-    > (jl-parse-file "<filename>")
+By default, Julia uses [JuliaSyntax.jl](https://github.com/JuliaLang/JuliaSyntax.jl) to produce the
+AST. Historically, it used a small lisp program written in femtolisp, the source-code for which is
+distributed inside Julia in [src/flisp](https://github.com/JuliaLang/julia/tree/master/src/flisp).
+If the `JULIA_USE_FLISP_PARSER` environment variable is set to `1`, the old parser will be used
+instead.
 
 ## [Macro Expansion](@id dev-macro-expansion)