The first line returned by `aspell` isn't always an identification
string, it can also be an error.
This commit prevents the first line from being ignored in any case,
and allows errors to be reported consistently.
Related to #3330
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
