feat(core): matcher_spelling.rs and docs for matcher.rs (fix #558)

[jnd-au]

Please see issue #558 for full context (Harper has spell check rules in both ./harper-core/src/linting/spell_check.rs and matcher.rs but the latter is undocumented and lacks a fine-grained configuration option to disable it, yet users may wish to disable it due to conflicts with their editors’ local/user spelling checker). This PR:

1. Documents the spell check message in matcher.rs (“Did you mean”).
2. Splits matcher.rs into spelling and non-spelling rules, using distinct messages that match the documentation.
3. Adds a configuration option matcher_spelling so that the spelling rules in matcher.rs can be disabled while keeping its grammar rules enabled.

This is a small tactical workaround, not a perfect solution, as the hard-coded matcher.rs is to be removed in a future release. A minor residual inconvenience is that users who have configured matcher=false and spell_check=false (the existing options) will see matcher spelling messages until they set matcher_spelling=false too (the new option). However, given the absence of existing GitHub issues and PRs, few users may be currently impacted.