fix(language-service): Improve signature selection by finding exact match

[kyliau]

The function signature selection algorithm is totally naive. It'd
unconditionally pick the first signature if there are multiple
overloads. This commit improves the algorithm by returning an exact
match if one exists.

PR Checklist

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PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
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• Other... Please describe:

What is the current behavior?

Issue Number: N/A

What is the new behavior?

Does this PR introduce a breaking change?

• Yes
• No

Other information

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