feat: convert spelling check to a standard check targets

[apskhem]

Description

Related Issue(s)

Closes #115

Description of Changes

Make spelling check conform to check target pattern

Please confirm the following checks

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream module

[Mr-Leshiy]

BTW, I think we should add this check target to the separate Earthfile, for example to the earthly/cspell/Earthfile.
So the idea is to run spell checks and markdown checks separately in different job, rather than in one as we will put it under the similar check target inside the root Earthfile.
Because if we will put it under the same target like this

check:
    BUILD +check-spelling
    BUILD +check-markdown

if spell check will fail, markdown check will not even run, what we don't want

@stevenj

[stevenj]

LGTM

Will be resolved in a follow up task