Enable EmmyLua's assign-type-mismatch diagnostic after correcting the
runtime annotations and assignments it flags.

Correct option, tuple, and return types, and use separate locals when a
value changes representation. Add guards and narrow casts where needed
so the runtime passes the stricter check. Regenerate the affected docs.

AI-assisted

Problem:
vim.hl.range() computes a segment for every selected line, then discards
all but the outer endpoints for characterwise and linewise highlights.
This makes highlighting large yanks and puts unnecessarily expensive.

Solution:
Use getregionpos() with bounds=true for these highlights. Also use bounds
in the keymap example that only needs the first and last selected lines.

AI-assisted

Problem:
Invalid `modes` parameter for `vim.keymap.del()` reports wrong parameter
name `"mode"`.

Solution:
Report correct parameter name `"mode"` instead.

Problem:
It is repetitive to map multiple keymaps to do the same thing. Here are some
cases where being able to do this would be useful:

    -- Visual movement for both j/k and down/up:
    vim.keymap.set({ 'n', 'x' }, { 'j', '<Down>' }, 'v:count == 0 ? "gj" : "j"', { expr = true })
    vim.keymap.set({ 'n', 'x' }, { 'k', '<Up>' }, 'v:count == 0 ? "gk" : "k"', { expr = true })
    -- Map multiple keys to `<Nop>` concisely:
    vim.keymap.set({ 'n', 'x' }, { '<Leader>', '<Localleader>', '<CR>' }, '<Nop>')
    -- Remove multiple keymaps at once:
    vim.keymap.del('n', { 'gri', 'grn', 'grr' })

Solution:
Support the `lhs` of `vim.keymap.set()` and `vim.keymap.del()` being a table, in
the same way that `modes` can be.

Problem:
`vim.keymap.*.Opts.buf` allows `boolean` aliases for more widely
used `integer?` values, `true` -> `0` and `false` -> `nil`. This
conversion is unnecessary and can be handled at call sites.

Solution:
As a follow-up to deprecating the `buffer` option, drop support for
boolean values for the new `buf` option. The deprecated `buffer`
continues to support booleans for backward compatibility.

The `buffer` option remains functional but is now undocumented.
Providing both will raise an error. Since providing `buf` was disallowed
before, there is no code that will break due to using `buffer` alongside
`buf`.

Problem: Unlike `nvim_keymap_set`, `vim.keymap.set` uses the non-negated
`remap` instead of `:set`'s `noremap`, but the documentation for this
got lost sometime before Nvim 0.10.

Solution: Restore the lost documentation and make it more explicit.

Problem: vim.keymap.del has 'modes' as it's first argument while vim.keymap.set
has 'mode' as it's first argument despite both 'mode' and 'modes' taking in the
same type input of String or String[].

Solution: Updated vim.keymap.set docs to refer to it's first argument
as 'modes'.