feat: file extensions opt-in and lookup

[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

resolves #4301

Todo:

• tests
• docs
• accept string option? (can also be given as single array value instead ['.js']
• how do extension-less files (currently) work? what would the option be? (empty string?)

Initially I thought to implement a requireFileExtensions (boolean) option, but I thought it might be better to re-use the same option for the extension lookup , which can also be specified now. similar to webpack resolve extensions

false (default)
currently an opt-in, and therefore non-breaking. in that case the internal default extensions would be used, which are currently ['.mjs', '.js'].

true
all given file paths are expected to have a file extension. they are required.

string[] e.g. ['.js'], ['.js', '.mjs']
replaces the default extensions, which also include the order of file lookups, for example ['.js', '.mjs] would first look for .js files, then for .mjs files. it also means that file extensions are not required, but still allowed.

personally I would strongly suggest to make this option an opt-out (default: true) for v3, as node.js, the web, and other engines (nginx, moddable etc.) work the same way (including I believe webpack). rollup seems to be the only one standing out.

[codecov]

Codecov Report

Merging #4303 (4a9140e) into master (2e91c85) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #4303   +/-   ##
=======================================
  Coverage   98.45%   98.45%           
=======================================
  Files         205      205           
  Lines        7311     7321   +10     
  Branches     2082     2085    +3     
=======================================
+ Hits         7198     7208   +10     
  Misses         55       55           
  Partials       58       58           

Impacted Files	Coverage Δ
src/utils/options/mergeOptions.ts	100.00% <ø> (ø)
src/utils/timers.ts	56.71% <ø> (ø)
src/ModuleLoader.ts	100.00% <100.00%> (ø)
src/utils/options/normalizeInputOptions.ts	100.00% <100.00%> (ø)
src/utils/resolveId.ts	95.65% <100.00%> (+0.91%)	⬆️

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[lukastaegert]

@dnalborczyk what is the state of this one? While I did not do a deep review yet, the proposal looks sound to me.