Problem:
Subdirectories like "visual", "insert", "normal" encourage people to
separate *related* tests for no good reason.  Typically the _mode_ is
not the relevant topic of a test (and when it is, _then_ create
an appropriate describe() or it()).

Solution:
- Delete the various `test/functional/<mode>/` subdirectories, move
  their tests to more meaningful topics.
- Rename `…/normal/` to `…/editor/`.
  - Move or merge `…/visual/*` and `…/insert/*` tests into here where
    appropriate.
- Rename `…/eval/` to `…/vimscript/`.
  - Move `…/viml/*` into here also.

* test(reorg): insert/* => editor/mode_insert_spec.lua
* test(reorg): cmdline/* => editor/mode_cmdline_spec.lua
* test(reorg): eval core tests => eval_spec.lua

Open external 'keywordprg' commands in a :terminal in a new tab. <Esc> is
mapped to stop the job and close the buffer.

Closes #2995
Closes #2761

In #8226 <A-x> and <M-x> were changed to behave like <Esc>x in insert
mode when no mapping exists. This commit backs out that change and
replaces it with a more general one that makes unmapped ALT and META
keypresses as <Esc>+char in all modes. This fixes an unnecessary and
confusing inconsistency between modes.

Problem:    Cannot switch from terminal window to previous tabpage.
Solution:   Make CTRL-W gT move to previous tabpage.
https://github.com/vim/vim/commit/882d02eeb571a13a502fe82a04c9eaffa630c294

Problem:    Cannot switch from terminal window to next tabpage.
Solution:   Make CTRL-W gt move to next tabpage.
https://github.com/vim/vim/commit/72e83c1ae535e2ebc35b114d34d0a811eb62b068

Traditionally, when navigating to a specific location from the middle of
the jumplist results in shifting the current location to the bottom of
the list and adding the new location after it.  This behavior is not
desireable to all users--see, for example
https://vi.stackexchange.com/questions/18344/how-to-change-jumplist-behavior.

Here, another jumplist behavior is introduced.  When jumpoptions (a new
option set added here) includes stack, the jumplist behaves like the
tagstack or like history in a web browser.  That is, when navigating to
a location from the middle of the jumplist

    2 first
    1 second
    0 third <-- current location
    1 fourth
    2 fifth

to a new location the locations after the current location in the jump
list are discarded

    2 first
    1 second
    0 third
            <-- current location

The result is that when moving forward from that location, the new
location will be appended to the jumplist:

    3 first
    2 second
    1 third
    0 new

If the new location is the same

  new == second

as some previous (but not immediately prior) entry in the jumplist,

    2 first
    1 second
    0 third <-- current location
    1 fourth
    2 fifth

both occurrences preserved

    3 first
    2 second
    1 third
    0 second (new)

when moving forward from that location.

It would be desireable to go farther and, when the new location is the
same as the location that is currently next in the jumplist,

    new == fourth

make the result of navigating to the new location by jumping (e.g. 50gg)
be the same as moving forward in the jumplist

    2 first
    1 second
    0 third
    1 new <-- current location
    2 fifth

and simply increment the jumplist index.  That change is NOT part of
this patch because it would require passing the new cursor location to
the function (setpcmark) from all of its callees.  That in turn would
require those callees to know *before* calling what the new cursor
location is, which do they do not currently.

- nvim_buf_get_extmarks: rename "amount" => "limit"
- rename `set_extmark_index_from_obj`

Eliminate `expect_err` in favor of `pcall_err` + `eq` or `matches`.