Always `emit()` nodes with zero children, don't `bump()` them

[c42f]

Cleanup weird cases where the parser would bump() interior nodes into the output stream rather than emit()ing them. Emitting all interior nodes explicitly means we can remove some special cases which occurred during tree building.

As part of this, fix some errors converting broken expressions like x var"y" from SyntaxNode to Expr.

Fixes #330

[codecov[bot]]

Codecov Report

Merging #371 (295b422) into main (96137ed) will increase coverage by 0.02%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #371      +/-   ##
==========================================
+ Coverage   96.56%   96.58%   +0.02%     
==========================================
  Files          14       14              
  Lines        4161     4160       -1     
==========================================
  Hits         4018     4018              
+ Misses        143      142       -1     

Files	Coverage Δ
src/expr.jl	99.69% <100.00%> (-0.01%)	:arrow_down:
src/literal_parsing.jl	98.78% <100.00%> (+0.42%)	:arrow_up:
src/parse_stream.jl	93.87% <100.00%> (-0.02%)	:arrow_down:
src/parser.jl	98.10% <100.00%> (+0.06%)	:arrow_up:
src/syntax_tree.jl	95.27% <100.00%> (-0.10%)	:arrow_down:

... and 1 file with indirect coverage changes