feat: add check for proper snake_case usage

[simojoe]

This pull request adds a new rule to wdl-grammar.

• Rule Name: snake_case
• Rule Kind: Lint warning
• Rule Code: v1::W006
• Packages: wdl-grammar

Describe the rules you have implemented and link to any relevant issues.

This rule enforces snake case on workflows, tasks and variables names.

I had to modify other parts of the project to create this PR :

• I changed the to_snake_case crate to the convert_case crate that will handle the same task (and more once the PascalCase rule gets implemented)
• I added the task_name rule to mirror the workflow_name rule. This allows for a cleaner identification of the task names for this lint rule.

This rule has been created with an Array of rules to check which should be snake cased. This is to enable potential opt-in or opt-out of the various elements from the snake_case and PascalCase rules.

One caveat is that this rule does not check the imports (can we properly type them to differentiate the struct from the rest?)

Before submitting this PR, please make sure:

• You have added a few sentences describing the PR here.
• You have added yourself or the appropriate individual as the assignee.
• You have added at least one relevant code reviewer to the PR.
• Your code builds clean without any errors or warnings.
• You have added an entry to the relevant CHANGELOG.md (see
  "keep a changelog" for more information).
• Your commit messages follow the conventional commit style.
• Your changes are squashed into a single commit (unless there is a really
  good, articulated reason as to why there shouldn be more than one).

Rule specific checks:

• You have added the rule as an entry within the the package-specific rule
  tables (wdl-ast/src/v1.rs for AST-based rules and
  wdl-grammar/src/v1.rs for parse tree-based rules).
• You have added the rule as an entry within the the global rule
  table at RULES.md.
• You have added the rule to the appropriate fn rules().
  • Validation rules added to wdl-ast should be added to fn rules() within
    wdl-ast/src/v1/validation.rs.
  • Lint rules added to wdl-ast should be added to fn rules() within wdl-ast/src/v1/lint.rs.
  • Validation rules added to wdl-grammar should be added to fn rules() within
    wdl-grammar/src/v1/validation.rs.
  • Lint rules added to wdl-grammar should be added to fn rules() within
    wdl-grammar/src/v1/lint.rs.
• You have added a test that covers every possible setting for the rule
  within the file where the rule is implemented.

[a-frantz]

Hi @simojoe! Thanks for the contribution! This PR is going to wait till we conclude the voting on #17.

(Our internal team has voted in concordance with how you've implemented things, but we want to get an idea of what the community is doing before cementing any decisions.)

[claymcleod]

I think this looks really good! Looking at the results of the discussion above, I think this is probably going to be a fine baseline rule that we modify over time. As such, I gave some (rather picky) thoughts here.