Support regex back references

[steve02081504]

Description

PS C:\Users\steve02081504\workstation\fount> typos
TOML parse error at line 22, column 20
   |
22 | extend-ignore-re = [
   |                    ^
regex parse error:
    (\s*)switch[^\n]*locales[^\s]\{\1\}
                                   ^^
error: backreferences are not supported

wanna a way to get more powerful regs :)

[epage]

I am reticent to support back references and lookaround. Part of this is an implementation detail: we are using regex which is a battle hardened and fast regex engine. I just looked at one that supports back references and it has two engines, maintained by different people, and i worry about the consistency between them. Part of this is that steering people away from these is good as people can easily make typos unnecessarily slow and it might be good to consider other solutions.