refactor(mrml-core): use a single MrmlParser structure with a visitor pattern

[jdrouet]

Todo

• increase coverage
• refactor to remove duplicates
• improve error messages to always give span

[codecov]

Codecov Report

Merging #317 (08590cf) into main (40e74db) will increase coverage by 0.73%.
The diff coverage is 94.82%.

@@            Coverage Diff             @@
##             main     #317      +/-   ##
==========================================
+ Coverage   92.39%   93.12%   +0.73%     
==========================================
  Files         222      230       +8     
  Lines       11880    12115     +235     
==========================================
+ Hits        10976    11282     +306     
+ Misses        904      833      -71     

Files Changed	Coverage Δ
packages/mrml-core/src/mj_accordion/children.rs	100.00% <ø> (ø)
...ges/mrml-core/src/mj_accordion_element/children.rs	66.66% <ø> (-25.65%)	⬇️
packages/mrml-core/src/mj_accordion_text/mod.rs	100.00% <ø> (ø)
packages/mrml-core/src/mj_accordion_title/mod.rs	100.00% <ø> (ø)
packages/mrml-core/src/mj_attributes/children.rs	100.00% <ø> (ø)
packages/mrml-core/src/mj_attributes/mod.rs	100.00% <ø> (ø)
packages/mrml-core/src/mj_attributes_all/mod.rs	100.00% <ø> (ø)
packages/mrml-core/src/mj_body/children.rs	96.87% <ø> (ø)
packages/mrml-core/src/mj_body/mod.rs	100.00% <ø> (ø)
packages/mrml-core/src/mj_breakpoint/mod.rs	100.00% <ø> (ø)
... and 73 more

... and 1 file with indirect coverage changes