new: nim -e:cmd to run a command directly; also fixes #15731 (#15687)

* new: `nim -i cmd`
* rename -i to -e (for eval); consistent with majority of other programing languages
* `nim e -e:cmd` now works; bugfix: `echo cmd | nim e -` now works
* honor --betterRun
* address comments
* --eval alias for -e (replaces undocumented --eval which was a noop)
* --eval now defaults to e (nimscript) instead of r
* address comment: remove -e, only keep --eval
* address comment
* fixup
* Update compiler/nimconf.nim

Co-authored-by: Andreas Rumpf <rumpf_a@web.de>

doc/basicopt.txt

@@ -31,6 +31,9 @@ Options:
   --app:console|gui|lib|staticlib
                             generate a console app|GUI app|DLL|static library
   -r, --run                 run the compiled program with given arguments
+  --eval:cmd                evaluates nim code directly; e.g.: `nim --eval:"echo 1"`
+                            defaults to `e` (nimscript) but customizable:
+                            `nim r --eval:'for a in stdin.lines: echo a'`
   --fullhelp                show all command line switches
   -h, --help                show this help
   -v, --version             show detailed version information