Missing overwrite mode

### Describe the issue you are having
It is often useful to overwrite, rather than insert, text. The IDE can do it. Might also work to make overstrikes, like the IDE does.

### How do you reproduce the issue?
Press Ins then position text caret in existing text and type; new text is inserted without replacing existing text.

### Paste the contents of Help → About (Shift+F1)
```
IDE:
Version: 4.5.3748
Electron: 19.0.4
Chrome: 102.0.5005.63
Node: 16.14.2
Platform: Win32
Date: 2022-08-19 14:40:20 +0200
Git commit: 6c723cbd2b2f29276d5c2d6728e99ab47145ac33
Preferences:{
"colourScheme":"Nord",
"colourSchemes":"[{\"name\":\"Default Neo\",\"theme\":\"light\",\"styles\":\"asgn=fg:00f com=fg:088 dfn=fg:00f diam=fg:00f err=fg:f00,U fn=fg:008 idm=fg:008 kw=fg:800 lnum=fg:008,bg:f,bgo:0 mod=bg:7,bgo:0.25 mtch=bg:ff8,bgo:0.5 norm=bg:#ffffff,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: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:#993333,bgo:1\"},{\"name\":\"Kazimir Malevich Neo\",\"theme\":\"light\",\"styles\":\"norm=bg:f,bgo:1, dc=bg:#993333 err=U\"},{\"name\":\"Albrecht Dürer Neo\",\"theme\":\"light\",\"styles\":\"com=I diam=B err=fg:0,bg:1,bgo:0.5,B,I,U glb=I kw=B lnum=bg:f,bgo:0 mod=bg:7,bgo:0.25 mtch=bg:c,bgo:0.5 norm=bg:f,bgo:1 ns=fg:8 num=fg:8 quad=fg:8 srch=bg:c,bgo:0.5 str=fg:8 tc=bg:e,bgo:1 tcpe=bg:dadada,bgo:1 zld=fg:8 vtt=bc:aaa dc=bg:#993333,bgo:1\"},{\"name\":\"Francisco Goya Neo\",\"theme\":\"dark\",\"styles\":\"asgn=fg:ff0 com=fg:b,I cur=bc:f00 dfn2=fg:eb4 dfn3=fg:c79 dfn4=fg:cd0 dfn5=fg:a0d dfn=fg:a7b diam=fg:ff0 err=fg:f00,bg:822,bgo:0.5,B,U fn=fg:0f0 idm=fg:0f0 glb=B kw=fg:aa2 lbl=U,bg:642,bgo:0.5 lnum=fg:b94,bg:010,bgo:0 mod=bg:7,bgo:0.5 mtch=fg:0,bg:0,bgo:0 norm=fg:9c7,bg:0,bgo:1 num=fg:a8b op1=fg:d95 op2=fg:fd6 sel=bg:048,bgo:0.5 semi=fg:8 sqbr=fg:8 srch=bg:b96,bgo:0.75,fg:0 str=fg:dae tc=bg:1,bgo:1 tcpe=bg:2,bgo:1 zld=fg:d9f,B scmd=fg:0ff ucmd=fg:f80,B vtip=bg:733,fg:ff0,bgo:1,bc:900 vtt=bc:f80 ca=bg:828282,bgo:1,fg:0f0 cm=bg:0,bgo:1,fg:0f0 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:900,bgo:1\"}]",
"confirmations":"{\"SaveFileOptionsExtension:.aplf\":100}",
"dbg":"1",
"indentOnOpen":"0",
"kbdLocale":"en_GB",
"keys":"{\"BK\":[\"Ctrl-Shift-Backspace\",\"Up\"],\"QCP\":[\"Ctrl-Shift-P\"],\"PF2\":[\"Shift-Up\"]}",
"lbarOrder":"← +-×÷*⍟⌹○!? |⌈⌊⊥⊤⊣⊢ =≠≤<>≥≡≢ ∨∧⍲⍱ ↑↓⊂⊃⊆⌷⍋⍒ ⍳⍸∊⍷∪∩~ /\\⌿⍀ ,⍪⍴⌽⊖⍉ ¨⍨⍣.∘⍤⍥@ ⍞⎕⍠⌸⌺⌶⍎⍕ ⋄⍝→⍺⍵∇& ¯⍬ ",
"pfkeys":"[\"\",\"\",\"\",\"\",\"\",\"\",\"\",\"\",\"\",\"\",\"\",\"\",\"\",\"\",\"\",\"\",\"\",\"\",\"\",\"\",\"\",\"\",\"\",\"\",\"\",\"\",\"\",\"\",\"\",\"\",\"\",\"\",\"\",\"\",\"\",\"\",\"\",\"\",\"\",\"\",\"\",\"\",\"\",\"\",\"\",\"\",\"\",\"\",\"\"]"
}

Interpreter:
Version: 19.0.47152
Platform: Windows-64
Edition: Unicode/64
Date: May 14 2023 at 18:19:17

```