T.P.GroffChar: replaced `essentialEscapes` with `manEscapes`,
which includes all the escapes mentioned in the groff_man manual.
T.P.Writers.Groff: removed escapeCode; changed parameter on
escapeString from Bool to new type `EscapeMode`.
Rewrote `escapeString`.
- `--ascii` is now turned on automatically for man output, for
portability. All man output will be escaped to ASCII.
- In T.P.Writers.Groff, `escapeChar`, `escapeString`, and
`escapeCode` now take a boolean parameter that selects
ascii-only output. This is used by the Ms writer for
`--ascii`, instead of doing an extra pass after writing
the document.
- In ms output without `--ascii`, unicode is used whenever
possible (e.g. for double quotes).
- A few escapes are changed: e.g. `\[rs]` instead of `\\` for
backslash, and `\ga]` instead of `` \` `` for backtick.
(unexported module). These are used in both the man and ms
writers.
Moved groffEscape out of Text.Pandoc.Writers.Shared [cancels earlier
API change from adding it, which was after last release].
This fixes strong/code combination on man (should be `\f[CB]` not
`\f[BC]`), mentioned in #4973.
Updated tests.
Closes#4975.
