Ms writer: handle tables with multiple paragraphs.

Previously they overflowed the table cell width.
We now set line lengths per-cell and restore them
after the table has been written.

Closes #7288.

test/command/7288.md

@@ -0,0 +1,40 @@
+```
+% pandoc -f rst -t ms
+.. list-table::
+   :widths: 50 50
+   :header-rows: 1
+
+   * - Left
+     - Right
+   * - Long text that should be easy to break up into multiple lines
+     - Another long text that should be easy to break up into multiple lines
+
+       Bar
+^D
+.PP
+.na
+.nr LLold \n[LL]
+.TS
+delim(@@) tab(	);
+lw(35.0n) lw(35.0n).
+T{
+Left
+T}	T{
+Right
+T}
+_
+T{
+.nr LL 35.0n
+.LP
+Long text that should be easy to break up into multiple lines
+T}	T{
+.nr LL 35.0n
+.PP
+Another long text that should be easy to break up into multiple lines
+.PP
+Bar
+T}
+.TE
+.nr LL \n[LLold]
+.ad
+```