Use more restrictive types

[dnalborczyk]

This PR contains:

• bugfix
• feature
• refactor
• documentation
• other

Are tests included?

• yes (bugfixes and features will not be merged without tests)
• no

Breaking Changes?

• yes (breaking changes will not be merged unless absolutely necessary)
• no

List any relevant issue numbers:

Description

[codecov]

Codecov Report

Merging #4782 (1f05bf1) into master (023ed0f) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #4782      +/-   ##
==========================================
- Coverage   99.03%   99.03%   -0.01%     
==========================================
  Files         217      217              
  Lines        7774     7773       -1     
  Branches     2159     2158       -1     
==========================================
- Hits         7699     7698       -1     
  Misses         24       24              
  Partials       51       51              

Impacted Files	Coverage Δ
src/Bundle.ts	100.00% <ø> (ø)
src/ast/scopes/ClassBodyScope.ts	100.00% <ø> (ø)
src/ast/scopes/ModuleScope.ts	100.00% <ø> (ø)
src/ast/scopes/ParameterScope.ts	100.00% <ø> (ø)
src/ast/variables/ExportShimVariable.ts	100.00% <ø> (ø)
src/ast/variables/ExternalVariable.ts	100.00% <ø> (ø)
src/ast/variables/NamespaceVariable.ts	100.00% <ø> (ø)
src/ast/variables/SyntheticNamedExportVariable.ts	100.00% <ø> (ø)
src/finalisers/cjs.ts	100.00% <ø> (ø)
src/finalisers/es.ts	100.00% <ø> (ø)
... and 6 more

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