provide some markdownish string formatting

[JanMarvin]

We could provide some interface to format strings. For example:

# for this example I just needed a file that already has inlinestrings
xlsxFile <- system.file("extdata", "inline_str.xlsx", package = "openxlsx2")
wb2 <- loadWorkbook(xlsxFile)
cc <- wb2$worksheets[[1]]$sheet_data$cc

# formatted strings (inline or shared) look like this:
# si/is + r node:
# rPr node defining the style used
# t node for the text
# rPr and t can repeat
# closing r + si/is
cc$is[1] <- paste0("<is><r><rPr><b/><i/><u/></rPr><t>PairedIndex</t></r><r><rPr><b/><i/><u/><sz val=\"11\"/><color theme=\"4\"/><rFont val=\"Calibri\"/><family val=\"2\"/><scheme val=\"minor\"/></rPr><t xml:space=\"preserve\"> text.</t></r></is>")

wb2$worksheets[[1]]$sheet_data$cc <- cc

wb_open(wb2)   

Certain features such as new line foo\nbar require the iirc column attribute wrap to be true.

[JanMarvin]

wrap text is in cellstyle