feat: support enum type definition in sheet

[wenchy]

• close protogen: propose enum type definition in a sheet #18
• close tableauc: inject build-time variables #11

TODO

• importer(TopN): for special type definition sheet.
• confgen config: no need to exclude common proto files: conf.excludedProtoFiles
• version: pretty print of version info

[codecov]

Codecov Report

Merging #54 (d438758) into master (ee4694a) will increase coverage by 2.65%.
The diff coverage is 37.60%.

@@            Coverage Diff             @@
##           master      #54      +/-   ##
==========================================
+ Coverage   31.15%   33.80%   +2.65%     
==========================================
  Files          25       28       +3     
  Lines        3451     3692     +241     
==========================================
+ Hits         1075     1248     +173     
- Misses       2289     2345      +56     
- Partials       87       99      +12     

Flag	Coverage Δ
unittests	33.80% <37.60%> (+2.65%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
internal/confgen/prop/refer.go	16.90% <0.00%> (ø)
internal/fs/util.go	1.61% <0.00%> (+1.61%)	⬆️
internal/importer/options.go	68.00% <ø> (ø)
internal/protogen/parser.go	7.43% <0.00%> (-0.02%)	⬇️
internal/protogen/exporter.go	17.72% <5.40%> (+6.38%)	⬆️
internal/xproto/build.go	44.23% <20.00%> (-0.77%)	⬇️
internal/protogen/protogen.go	14.40% <21.12%> (+14.40%)	⬆️
internal/protogen/util.go	41.42% <41.42%> (ø)
internal/importer/csv.go	43.75% <41.83%> (-9.99%)	⬇️
internal/importer/excel.go	33.07% <45.09%> (+5.54%)	⬆️
... and 6 more

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[huieric]

LGTM.