Xonsh glow-ups

Move xonsh header/footer to separate files
Add gregpy as the default Python version used in xonsh
Add Nix profile to PATH in xonsh ahead of tmux path mangling to allow
python3 with modules to be discovered

home/xonsh_header.xsh

@@ -0,0 +1,17 @@
+# vim: set ft=python:
+if  '__NIX_DARWIN_SET_ENVIRONMENT_DONE' not in ${...} or not $__NIX_DARWIN_SET_ENVIRONMENT_DONE:
+    source-bash /etc/bashrc
+
+# set -e == $RAISE_SUBPROC_ERROR = True
+# set -x == trace on; $XONSH_TRACE_SUBPROC = True
+# $? == _.rtn
+
+import os
+
+xontrib load direnv
+
+# Insert to the front, because when we spawn xonsh in tmux we have raw python3
+# paths added before these. That ends up screwing with finding the python3 version
+# with all of our dependencies that we want
+$PATH.insert(0, "${config.home.homeDirectory}/.nix-profile/bin")
+$PATH.insert(0, "${config.home.homeDirectory}/src/bin")