lvjr

Clean up begin/end pairs and scratch variables

2

There are quite a few functions in `tabularray` enclose their body with `\group_begin:`/`\group_end:` pairs, such as ```latex \cs_new_protected:Npn \__tblr_get_vcell_and_sizes:NN #1 #2 { \group_begin: \vbox_set:Nn \l_tmpb_box { \__tblr_make_vcell_text:NN #1 #2 }...

With outer option `transpose`, we could input cell text column by column. ```latex \begin{tblr}[transpose]{} a & b & c \\ d & e & f \\ g & h &...

Actually, you can pass a negative value to `baseline` key to make a `tblr` table align at some hborder. This is undocumented because `tabularray` doesn't align the center of border...

From discussion https://github.com/lvjr/tabularray/discussions/537 . Sometimes users may want to add borders to or remove borders from some cells only. Therefore some new keys may be added to the optional argument...

We can copy `wiki/ChangeLog.md` to `CHANGELOG.md`, update it based on [Keep a Changelog](https://keepachangelog.com) format, then read it into the manual with `markdown` package.