Added symbol for mils.

[jjcazau]

Added missing unit symbol for mils which is "th". "mil" is also used, mainly in the US.

https://simple.wikipedia.org/wiki/Thou_(unit)

[codecov-commenter]

Codecov Report

Base: 99.77% // Head: 99.77% // No change to project coverage :thumbsup:

Coverage data is based on head (ef839f5) compared to base (46020b1). Patch has no changes to coverable lines.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #34 +/- ## ======================================= Coverage 99.77% 99.77% ======================================= Files 31 31 Lines 905 905 ======================================= Hits 903 903 Misses 2 2 ``` | [Impacted Files](https://codecov.io/gh/ferraridamiano/units_converter/pull/34?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Damiano+Ferrari) | Coverage Δ | | |---|---|---| | [lib/properties/length.dart](https://codecov.io/gh/ferraridamiano/units_converter/pull/34?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Damiano+Ferrari#diff-bGliL3Byb3BlcnRpZXMvbGVuZ3RoLmRhcnQ=) | `100.00% <ø> (ø)` | | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Damiano+Ferrari). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Damiano+Ferrari)

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[ferraridamiano]

Thank you!