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>

compiler/vm.nim

@@ -2114,6 +2114,9 @@ proc evalStmt*(c: PCtx, n: PNode) =
     discard execute(c, start)
 
 proc evalExpr*(c: PCtx, n: PNode): PNode =
+  # deadcode
+  # `nim --eval:"expr"` might've used it at some point for idetools; could
+  # be revived for nimsuggest
   let n = transformExpr(c.graph, c.idgen, c.module, n)
   let start = genExpr(c, n)
   assert c.code[start].opcode != opcEof