Problem:
`compute_hash()` reconstructs buffer contents with `fileformat` set to
`mac` using CRLF (`dos`) line endings. The resulting hash does not match
the file's bytes.

Solution:
Define the line ending for every supported file format. Use that mapping
when reconstructing buffer contents for hashing.

Problem:
Malicious filename can execute code because of ":" cmdline expansion.

Solution:
Use `fnameescape()`.

fix https://github.com/neovim/neovim/issues/39914

Problem:
We should not use "\" (backslashes) except where absolutely required.
See references in https://github.com/neovim/neovim/pull/37729

Solution:
There is no reason to use "\" slashes in the trust db, so don't.

Problem:
`vim.secure.trust()` hashes an unchanged empty buffer as
a newline, so trusting an empty file by buffer never works.

Solution:
Hash unchanged empty-buffers `''` so buffer-based
trust matches the on-disk empty file.

gen_vimdoc.lua: In prepare for the upcoming release, comment-out the
"Experimental" warning for prerelease features.

`:trust` command calculated SHA-256 on file content reading it as a
text. While it doesn't matter on Unices, on Windows hash was calculated
incorectly. SHA-256 for buffer content was calculated fine though.

After this fix hashes in `%LOCALAPPDATA%/nvim-data/trust` are the same
as in output of `sha256sum -t`.

Problem:
It's relatively easy to mispress key `a` to (a)llow arbitrary execution
of 'exrc' files. #35050

Solution:
- For exrc files (not directories), remove "allow" menu item.
  Require the user to "view" and then explicitly `:trust` the file.

Type fixes caught by emmylua