Improved Bash support

[Xophmeister]

Preliminary Bash formatting support was tracked in #138. There are a number of improvements that could be made -- some of which are necessary to consider the Bash formatting rules "supported" -- which we track here:

(Legend: 💥 Critical; ⚠️ Desirable; 💡 Nice-to-Have)

• 💥 Heredocs are affected by the indentation level #200
• 💥 Invalid syntax generated in assignment of command substitutions #201
• ⚠️ One or both of:
  • Ability to add line continuations in multi-line contexts #172
  • Discussion: Delimited softlines #175
• ⚠️ Blank lines between comments are not always preserved #202
• ⚠️ Errant blank lines added after runs of statements #203
• 💡 [Bash] Convert (simple_expansion) into (expansion) #187
• 💡 Alignment blocks #170