`selection` variable in menu

### Describe the issue you are having
I'd like to add menu items to search for what's currently selected using an online service. It'd e.g. be convenient to add
```
&Help
&APLcart =https://aplcart.info?q={selection}
```

### Paste the contents of Help → About (Shift+F1)
```
IDE:
Version: 4.3.3438
Platform: Win32
Date: 2019-09-03 14:09:43 +0200
Git commit: cc9cc6b4c5000c8033146a1731bc376d09f30ee1
Preferences:{
"colourScheme":"Default (1)",
"colourSchemes":"[{\"name\":\"Default (1)\",\"theme\":\"light\",\"styles\":\"asgn=fg:00f com=fg:088 dfn=fg:00f diam=fg:00f err=fg:f00 fn=fg:008 idm=fg:008 kw=fg:800 lnum=fg:008,bg:f,bgo:0 mod=bgo:0.25 mtch=bg:ff8,bgo:0.5 norm=bg:f,bgo:1 ns=fg:8 num=fg:8 op1=fg:00f op2=fg:00f par=fg:00f quad=fg:808 qdl=fg:c0c sel=bg:48e,bgo:0.5 semi=fg:00f sqbr=fg:00f srch=bg:f80,bgo:0.5 str=fg:088 tc=bg:d,bgo:1 tcpe=bg:c8c8c8,bgo:1 trad=fg:8 var=fg:8 zld=fg:008 scmd=fg:00f ucmd=fg:00f vtt=bg:ff0 ca=bg:828282,bgo:1,fg:0f0 cm=bg:0,bgo:0.1,fg:080 cv=bg:f,bgo:1,fg:0 cvv=bg:0,bgo:1,fg:0ff ma=bg:828282,bgo:1,fg:0ff na=bg:828282,bgo:1,fg:f qor=bg:f00,bgo:1,fg:f dc=bg:e2d3bb,bgo:1\"}]",
"indentOnOpen":"0",
"kbdLocale":"en_US",
"keys":"{\"DMK\":[\"Ctrl-K\"],\"SI\":[\"Ctrl-C\"],\"VAL\":[\"Ctrl-Alt-V\"]}",
"menu":"# see below for syntax\nDyalog {mac}\n About Dyalog =ABT\n -\n Preferences =PRF\n - \n &Quit =QIT\n&File {!browser}\n &Open... =OWS\n &New Session =NEW\n &Connect... =CNC\n - {!mac}\n &Quit =QIT {!mac}\n&Edit\n Undo =UND {!browser}\n Redo =RDO {!browser}\n - {!browser}\n Cut =CT {!browser}\n Copy =CP {!browser}\n Paste =PT {!browser}\n Select All =SA {mac}\n - {!mac&&!browser}\n Preferences =PRF {!mac}\n&View\n Show Language Bar =LBR\n Show Status Bar =SBR\n Show Workspace Explorer =WSE\n Show Debug =DBG\n Line Wrapping in Session =WRP\n - {!browser}\n Stops =TVB\n Line Numbers =LN\n Outline =TVO\n - {!browser}\n Increase Font Size =ZMI {!browser}\n Decrease Font Size =ZMO {!browser}\n Reset Font Size =ZMR {!browser}\n - {!browser}\n Toggle Full Screen {!browser}\n&Window\n Close All Windows =CAW\n&Action\n Edit =ED\n Trace =TC\n -\n Clear all trace/stop/monitor =CAM\n Weak Interrupt =WI\n Strong Interrupt =SI\n&Help\n &APLcart =https://aplcart.info\n Dyalog Help =http://help.dyalog.com/\n Documentation Centre =http://dyalog.com/documentation.htm\n -\n Dyalog Website =http://dyalog.com/\n MyDyalog =https://my.dyalog.com/\n -\n Dyalog Forum =http://www.dyalog.com/forum\n - {!mac}\n About =ABT {!mac}\n\n# Syntax:\n# &x access key, alt+x\n# =CMD command code; some are special:\n# LBR WRP WSE render as checkboxes\n# =http://example.com/ open a URL\n# {} conditional display, a boolean expression\n# operators: && || ! ( )\n# variables: browser mac win\n# - separator (when alone)\n# # comment\n\n# The =PRF (\"Preferences\") menu item must be present.",
"otherExe":"C:\\Program Files\\Dyalog\\Dyalog APL-64 18.0 Unicode\\dyalog.exe",
"selectedExe":"C:\\Program Files\\Dyalog\\Dyalog APL-64 19.0 Unicode\\dyalog.exe"
}

Interpreter:
Version: 18.0.38756
Platform: Windows-64
Edition: Unicode/64
Date: Jul 15 2020 at 19:56:53

```