Feature flag for eventually disabling terminal workarounds

We've removed several terminal-specific workarounds but also added
some recently.  Most of the non-Apple issues have been reported
upstream.  Many of our workarounds were only meant to be temporary.
Some workarounds are unreliable and some can cause introduce other
problems.

Add a feature flag we can use later to let users turn off workarounds.

This doesn't disable anything yet, mostly because because despite being
off-by-default, this might surprise people who use "fish_features=all".
The fix would be "fish_features=all,no-omit-term-workarounds".
So we'd want a high degree of confidence.

For now we'll use this only with the next commit.

Closes #11819

doc_src/language.rst

@@ -2027,6 +2027,7 @@ You can see the current list of features via ``status features``::
     mark-prompt             on  4.0 write OSC 133 prompt markers to the terminal
     ignore-terminfo         on  4.1 do not look up $TERM in terminfo database
     query-term              on  4.1 query the TTY to enable extra functionality
+    omit-term-workarounds   off 4.3 skip workarounds for incompatible terminals
 
 Here is what they mean:
 
@@ -2041,6 +2042,7 @@ Here is what they mean:
 - ``query-term`` allows fish to query the terminal by writing escape sequences and reading the terminal's response.
   This enables features such as :ref:`scrolling <term-compat-cursor-position-report>`.
   If you use an incompatible terminal, you can -- for the time being -- work around it by running (once) ``set -Ua fish_features no-query-term``.
+- ``omit-term-workarounds`` prevents fish from trying to work around incompatible terminals.
 
 
 These changes are introduced off by default. They can be enabled on a per session basis::