runtime(dist): Make dist/vim.vim work properly when lacking vim9script support (vim/vim#13487)

`:return` cannot be used outside of `:function` (or `:def`) in older Vims
lacking Vim9script support or in Neovim, even when evaluation is being skipped
in the dead `:else` branch.

Instead, use the pattern described in `:h vim9-mix`, which uses `:finish` to end
script processing before it reaches the vim9script stuff.

https://github.com/vim/vim/commit/b2a4c110a5d13bc794f4eddb2e88a4e8fe9dfbea

Co-authored-by: Sean Dewar <seandewar@users.noreply.github.com>

runtime(dist): add legacy version for central vim library

Also, enable the zip and gzip plugins by default, unless those variables
were not explicitly set by the user.

related: vim/vim#13413

https://github.com/vim/vim/commit/4f174f0de90b52937ddaf1e6db98e9731930ff7c

Co-authored-by: Christian Brabandt <cb@256bit.org>

runtime(dist): centralize safe executable check and add vim library (vim/vim#13413)

Follow up to 816fbcc26 (patch 9.0.1833: [security] runtime file fixes,
2023-08-31) and f7ac0ef50 (runtime: don't execute external commands when
loading ftplugins, 2023-09-06).

This puts the logic for safe executable checks in a single place, by introducing
a central vim library, so all filetypes benefit from consistency.

Notable changes:
- dist#vim because the (autoload) namespace for a new runtime support
  library. Supporting functions should get documentation. It might make
  life easier for NeoVim devs to make the documentation a new file
  rather than cram it into existing files, though we may want
  cross-references to it somewhere…
- The gzip and zip plugins need to be opted into by enabling execution
  of those programs (or the global plugin_exec). This needs
  documentation or discussion.
- This fixes a bug in the zig plugin: code setting s:tmp_cwd was removed
  in f7ac0ef50 (runtime: don't execute external commands when loading
  ftplugins, 2023-09-06), but the variable was still referenced. Since
  the new function takes care of that automatically, the variable is no
  longer needed.

https://github.com/vim/vim/commit/cd8a3eaf5348feacfecab4b374b7ea4ce6a97422

Co-authored-by: D. Ben Knoble <ben.knoble+github@gmail.com>

Made obsolete by now graduated `filetype.lua` (enabled by default).

Note that changes or additions to the filetype detection still need to
be made through a PR to vim/vim as we port the _logic_ as well as tests.

vim-patch:9.0.0771: cannot always tell the difference beween tex and rexx files

Problem:    Cannot always tell the difference beween tex and rexx files.
Solution:   Recognize tex by a leading backslash. (Martin Tournoij,
            closes vim/vim#11380)
https://github.com/vim/vim/commit/bd053f894b0d7652928201faa68c53d1ce2acdc5

Problem:    Filetype of *.sil files not well detected.
Solution:   Inspect the file contents to guess the filetype.
https://github.com/vim/vim/commit/be807d582499acbe314ead3891481cba6ca136df

vim-patch:partial:48c3f4e0bff7

Update runtime files
https://github.com/vim/vim/commit/48c3f4e0bff7efd289a7001b68c777b6f89a7057

partially skip `options.txt` (needs 9.0.0138)

Problem:    Pattern for detecting bitbake files is not sufficient.
Solution:   Adjust the pattern. (Gregory Anders, closes vim/vim#10743)
https://github.com/vim/vim/commit/30e212dac1d29536883c36918a465a38d81d6413

vim-patch:9.0.0055: bitbake files are not detected

Problem:    Bitbake files are not detected.
Solution:   Add bitbake filetype detection by file name and contents. (Gregory
            Anders, closes vim/vim#10697)
https://github.com/vim/vim/commit/fa49eb482729a5fe7da9c9a5ed8d14f68afa55c7

Problem:    Signature files not detected properly.
Solution:   Add a function to better detect signature files. (Doug Kearns)
https://github.com/vim/vim/commit/cdbfc6dbab1d63aa56af316d6b13e37939e7f7a8