Rename ENV variable JULIA_USE_NEW_PARSER -> JULIA_USE_FLISP_PARSER (#50595)

Closes #50470

Author: oscardssmith

HISTORY.md

@@ -4,7 +4,7 @@ New language features
 ---------------------
 
 * JuliaSyntax.jl is now used as the default parser, providing better diagnostics and faster
-  parsing. Set environment variable `JULIA_USE_NEW_PARSER` to `0` to switch back to the old
+  parsing. Set environment variable `JULIA_USE_FLISP_PARSER` to `1` to switch back to the old
   parser if necessary (and if you find this necessary, please file an issue) ([#46372]).
 * `⥺` (U+297A, `\leftarrowsubset`) and `⥷` (U+2977, `\leftarrowless`)
   may now be used as binary operators with arrow precedence. ([#45962])

base/Base.jl

@@ -604,7 +604,7 @@ function __init__()
     _require_world_age[] = get_world_counter()
     # Prevent spawned Julia process from getting stuck waiting on Tracy to connect.
     delete!(ENV, "JULIA_WAIT_FOR_TRACY")
-    if get_bool_env("JULIA_USE_NEW_PARSER", true) === true
+    if get_bool_env("JULIA_USE_FLISP_PARSER", false) === false
         JuliaSyntax.enable_in_core!()
     end
     nothing