implement plain syntax

[marihachi]

What

Resolve #101
Resolve #113

• 構文追加: <plain>構文
• API追加: type MfmPlain、PLAIN()
• API変更: parsePlain() -> parseSimple()
• API変更: type MfmPlainNode -> type MfmSimpleNode
• npm-scripts変更: parse-plain -> parse-simple

Why

Additional info (optional)

[codecov]

Codecov Report

Merging #114 (437b7d4) into develop (c2f166d) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop     #114   +/-   ##
========================================
  Coverage    99.62%   99.63%           
========================================
  Files            4        4           
  Lines          539      553   +14     
  Branches       107      110    +3     
========================================
+ Hits           537      551   +14     
  Misses           1        1           
  Partials         1        1           

Impacted Files	Coverage Δ
src/api.ts	100.00% <100.00%> (ø)
src/index.ts	100.00% <100.00%> (ø)
src/internal/util.ts	99.00% <100.00%> (+0.01%)	⬆️
src/node.ts	100.00% <100.00%> (ø)

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[syuilo]

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