This commit adds a `spell_lang` option that will be used by the
`spell` command when no language is passed to it directly.
Setting a buffer/window local option allows using `:spell` seamlessly
across several buffers, written in different languages.
Due to ambiguity in the POSIX standard, GNU and BSD versions of the `wc`
utility use slightly different whitespace conventions when formatting
their output [1]. When limiting the output to just counting the number
of bytes (as is done by Kakoune when calculating the length of words
for spell check highlighting), the BSD version of `wc -c` has some
additional leading whitespace:
gnu$ printf %s "test" | wc -c
4
bsd$ printf %s "test" | wc -c
4
This leading whitespace needs to be removed before defining the "region"
to highlight, or `set-option` will not be able to parse the given
`spell_regions` and will complain that there are "not enough elements
in tuple." In other words, the region `1.21+8|Error` on Linux ends up
looking like `1.21+ 8|Error` on macOS, which is invalid.
Removing the whitespace could be accomplished in a number of ways, but
using arithmetic expansion [2] is POSIX compliant and does not require
shelling out to another process.
[1]: https://unix.stackexchange.com/questions/205906/extra-space-with-counted-line-number
[2]: https://mywiki.wooledge.org/ArithmeticExpression
