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/msgs.nim

@@ -523,6 +523,7 @@ proc liMessage*(conf: ConfigRef; info: TLineInfo, msg: TMsgKind, arg: string,
   let s = if isRaw: arg else: getMessageStr(msg, arg)
   if not ignoreMsg:
     let loc = if info != unknownLineInfo: conf.toFileLineCol(info) & " " else: ""
+    # we could also show `conf.cmdInput` here for `projectIsCmd`
     var kindmsg = if kind.len > 0: KindFormat % kind else: ""
     if conf.structuredErrorHook != nil:
       conf.structuredErrorHook(conf, info, s & kindmsg, sev)