Disallow unbalanced chunk delimiters

[yihui]

Notice has been given for two years: https://yihui.org/en/2021/10/unbalanced-delimiters/

From the revdeps check, these packages will be affected (I have notified all maintainers):

• [x] https://github.com/Rdatatable/data.table/issues/5735
• [x] To be archived: https://cran.r-project.org/web/packages/abbyyR/index.html
• [x] https://github.com/stan-dev/loo/pull/232
• [x] https://github.com/TomKellyGenetics/leiden/pull/16
• [ ] https://cran.r-project.org/package=ensembleR
• [ ] https://cran.r-project.org/package=FSinR
• [x] https://cran.r-project.org/package=funmediation
• [ ] https://cran.r-project.org/package=liger
• [ ] https://cran.r-project.org/package=TRMF
• [ ] https://cran.r-project.org/package=microsamplingDesign
• [ ] https://cran.r-project.org/package=mmpf
• [ ] https://cran.r-project.org/package=rSEA
• [ ] https://cran.r-project.org/package=StructFDR