Add support for package extensions in `show` method

[hyrodium]

This PR adds support for package extensions by counting lines in the ext directory.

Before this PR

julia> using PackageAnalyzer

julia> analyze("https://github.com/hyrodium/BasicBSpline.jl")
PackageV1 BasicBSpline:
  * repo: https://github.com/hyrodium/BasicBSpline.jl
  * uuid: 4c5d9882-2acf-4ea4-9e48-968fd4518195
  * version: missing
  * is reachable: true
  * tree hash: 9b4f65180a415c9a61975b1dafaee2cdd07011d1
  * Julia code in `src`: 2444 lines
  * Julia code in `test`: 1828 lines (42.8% of `test` + `src`)
  * documentation in `docs`: 1237 lines (33.6% of `docs` + `src`)
  * documentation in README & docstrings: 348 lines (12.5% of README + `src`)
  * has license(s) in file: MIT
    * filename: LICENSE
    * OSI approved: true
  * has `docs/make.jl`: true
  * has `test/runtests.jl`: true
  * has continuous integration: true
    * GitHub Actions

After this PR

julia> using PackageAnalyzer

julia> analyze("https://github.com/hyrodium/BasicBSpline.jl")
PackageV1 BasicBSpline:
  * repo: https://github.com/hyrodium/BasicBSpline.jl
  * uuid: 4c5d9882-2acf-4ea4-9e48-968fd4518195
  * version: missing
  * is reachable: true
  * tree hash: 9b4f65180a415c9a61975b1dafaee2cdd07011d1
  * Julia code in `src`: 2444 lines
  * Julia code in `ext`: 161 lines (3.6% of `test` + `src` + `ext`)
  * Julia code in `test`: 1828 lines (41.2% of `test` + `src` + `ext`)
  * documentation in `docs`: 1237 lines (32.2% of `docs` + `src` + `ext`)
  * documentation in README & docstrings: 348 lines (12.5% of README + `src`)
  * has license(s) in file: MIT
    * filename: LICENSE
    * OSI approved: true
  * has `docs/make.jl`: true
  * has `test/runtests.jl`: true
  * has continuous integration: true
    * GitHub Actions

[hyrodium]

I think the CI failure on Julia 1.6 - windows-latest is not related to this PR.