refactor(build): update fortran standard to 2018

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 84.124%. Comparing base (516b61e) to head (846fcf8). Report is 4 commits behind head on develop.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #195 +/- ## ============================================= - Coverage 84.181% 84.124% -0.058% ============================================= Files 20 20 Lines 3496 3496 ============================================= - Hits 2943 2941 -2 - Misses 553 555 +2 ``` | [Files](https://app.codecov.io/gh/modflowpy/pymake/pull/195?dropdown=coverage&src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=modflowpy) | Coverage Δ | | |---|---|---| | [pymake/utils/\_compiler\_switches.py](https://app.codecov.io/gh/modflowpy/pymake/pull/195?src=pr&el=tree&filepath=pymake%2Futils%2F_compiler_switches.py&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=modflowpy#diff-cHltYWtlL3V0aWxzL19jb21waWxlcl9zd2l0Y2hlcy5weQ==) | `79.361% <ø> (ø)` | | | [pymake/utils/\_meson\_build.py](https://app.codecov.io/gh/modflowpy/pymake/pull/195?src=pr&el=tree&filepath=pymake%2Futils%2F_meson_build.py&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=modflowpy#diff-cHltYWtlL3V0aWxzL19tZXNvbl9idWlsZC5weQ==) | `90.076% <100.000%> (ø)` | | ... and [1 file with indirect coverage changes](https://app.codecov.io/gh/modflowpy/pymake/pull/195/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=modflowpy)

[mjreno]

Will reopen.