Add lint rule requireForeignKeyIndex

[k1LoW]

# .tbls.yml
lint:
  requireForeignKeyIndex:
    enabled: true
    exclude:
      - comments.user_id

[codecov]

Codecov Report

Merging #142 into master will decrease coverage by 0.78%.
The diff coverage is 68.42%.

@@            Coverage Diff             @@
##           master     #142      +/-   ##
==========================================
- Coverage   69.32%   68.53%   -0.79%     
==========================================
  Files          12       12              
  Lines        2657     2708      +51     
==========================================
+ Hits         1842     1856      +14     
- Misses        624      661      +37     
  Partials      191      191

Impacted Files	Coverage Δ
schema/schema.go	44.61% <ø> (ø)	⬆️
drivers/mssql/mssql.go	82.87% <ø> (ø)	⬆️
drivers/sqlite/sqlite.go	85.86% <100%> (ø)	⬆️
drivers/mysql/mysql.go	81.99% <100%> (+0.05%)	⬆️
drivers/postgres/postgres.go	70.76% <100%> (ø)	⬆️
config/lint.go	88.37% <66.66%> (-0.16%)	⬇️
datasource/datasource.go	20.85% <0%> (-19.02%)	⬇️

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