watch: implement interval

[maxer137]

This merge aims to add an interval parsing implementation for watch.

      -n, --interval seconds
              Specify update interval.  The command will not allow quicker than 0.1 second inter-
              val,  in  which the smaller values are converted. Both '.' and ',' work for any lo-
              cales. The WATCH_INTERVAL environment can be used to persistently set a non-default
              interval (following the same rules and formatting).

In watch.rs, a new function named parse_interval is added, which takes in a &str and returns a result with Duration or an error with ParseIntError.

The function will first see if the string contains either a '.' or a ',' character. If it does not, it attempts to parse it into seconds and return a duration with that many seconds.
Otherwise, it will attempt to split the string into the seconds and the remainder.
The remainder is then parsed and zero padded. This way we ensure 0.5 becomes 500000000 nanoseconds, and not 5.
If the string has a missing component (such as .5 or 5.) the function will assume 0.
Nanosecond strings that except 9 characters will first be checked if they are valid, then trimmed.
Finally, return the calculated duration or 100 milliseconds. Whichever is longer. This ensures the minimum interval of 0.1 seconds.

Other thoughts:

1. Currently, I have used "a".parse::<u8>()? to get rust to return a ParseIntError. This is obviously not ideal, but it seems ParseIntError is private to rust, and we'd need to make a custom return type instead. This is possible, but it seemed overkill for this scenario.
2. There may be a more efficient calculation for padding the nanoseconds. I used 10u32.pow() but maybe a while loops would be more efficient.
3. The WATCH_INTERVAL environment variable has not been implemented yet.

[codecov]

Codecov Report

Attention: Patch coverage is 49.12281% with 29 lines in your changes are missing coverage. Please review.

Project coverage is 49.22%. Comparing base (c5e7417) to head (7e93abd).
Report is 2 commits behind head on main.

Files	Patch %	Lines
src/uu/watch/src/watch.rs	16.12%	22 Missing and 4 partials ⚠️
tests/by-util/test_watch.rs	88.46%	0 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #48      +/-   ##
==========================================
+ Coverage   45.53%   49.22%   +3.68%     
==========================================
  Files          15       15              
  Lines        1803     1804       +1     
  Branches      240      244       +4     
==========================================
+ Hits          821      888      +67     
+ Misses        853      776      -77     
- Partials      129      140      +11     

Flag	Coverage Δ
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ubuntu_latest	49.22% <49.12%> (+3.81%)	⬆️
windows_latest	?

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[sylvestre]

could you please check stderr too here?
(at least with .contains, doesn't have to be a full match)

[maxer137]

Added in latest commit